SCOPE, OPTICS & BALLISTIC INFO & FAQs

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Frogman Ladue
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SCOPE, OPTICS & BALLISTIC INFO & FAQs

Postby Frogman Ladue » 01 Jun 2014, 21:48

Hey guys, cross posting "Scope Info" from a State Side Gun Forum. Albiet, info is in inch pattern an bias to US scope brands, it certainly covers all the basics.


There are often many posts or questions about riflescopes. In order to be of service, I have compiled this information thread to answer your questions. This will be an ongoing process and some data may change or be added - e.g. the spring-piston air rifle scope section is a new addition. I hope it covers most of the subjects that you might want to know about scopes and optics. While I do have extensive experience with hunting scopes gathered over the years, I mainly drew information from expert sources such as: John Barsness, Wayne Van Zwoll, and Ron Spomer - optics editors for RIFLE magazine, Jacob Gottfredson, optics editor for GUNS magazine, Todd Spotti in the IHMSA News, and other industry sources like Burris, Leupold, Bushnell, Simmons, SWFA and others, including fellow Moderator Onepoint who assisted tremendously in input and finding information. Certainly his optic expertise should be credited on this forum.
While I tried not to bring any personal prejudice into the thread, I own and therefore am most familiar with certain brands of scopes so some of that may show. I hope you find this information most profitable in any future scope purchase, the use of your current scopes, and you will learn something of value.


WHAT THE NUMBERS MEAN
A scope designation often is presented like this:
3-9x 40mm. Or 4x32mm
Broken down, in the first case this means a 3 power (the “x” for “times”) to 9 power magnification variable scope with a 40mm objective (front) lens. The size of the front lens indicates some light gathering value and helps calculate Exit Pupil (described later). The second case would be a 4 power - fixed power scope with a 32mm objective lens. Occasionally this designation might be followed by something like “AO”. This means it has an Adjustable Objective that can be focused at a certain range to eliminate parallax. This can be spotted by a series of yardage or meter numbers on an objective that can be turned to make the numbers correspond with a line.
As of late, you might see a scope described as a “30mm” 4-12x 42mm. This would indicate that the body of the scope follows the European pattern of a larger than the American standard 1 inch body diameter and would require larger 30mm rings. Some older cheap .22 rimfire scopes, and even ancient hunting scopes, were only ¾” or 7/8” in tube diameter.
So a 4.5-14x 50mm AO would be a 4 ½ to 14 power variable scope with a 50mm objective lens and an adjustable objective. A 1 inch tube (and ring) size would be assumed because there is no 30mm designation.

There are also a couple of other "letter" designations that may follow the numbers, and these also deal with a specific scope usage.
"XER" - Extended Eye Relief. "EER" - Extended Eye Relief. "IER" - Intermediate Eye Relief. There may be other brand-specific designations for a scope that has the eye relief is much longer than a regular scope and the scope is mounted further from the eye. These are used primarily in handgun applications, and some models as a "Scout" scope, which are mounted on a rifle forward of the receiver over the barrel area as popularized by Col Jeff Cooper for his "Scout" rifle concept.

TERMS & MISCELLANEOUS

EYE RELIEF (ER)
The distance a scope can be held away from the eye and still present the full field of view.
Some things to know about eye relief is that many, but not all, variable scopes sacrifice eye relief (or eye relief shrinks) as power increases. The greater the top-end power, the less eye relief at those settings. Those that do threaten to give you the infamous half-moon scope cut on your eyebrow during recoil. Aim for a minimum of 3 inches of ER at the .30-06 level of recoil, 3.5 inches with .300 magnums, and 4 inches would be better. Fixed power scopes have a fixed eye relief distance. Mounting position on the rifle in the rings can be affected by ER limitations.
You can measure eye relief in your scope by gently placing a small flashlight against the objective lens and shining it into the scope. Next you hold some flat object (your palm will do) about three inches behind the eyepiece. A dot of light will be "projected" on your palm. This is the exit pupil of your scope. Move your hand until the dot is as small and sharp as possible, and the distance from the rear of the scope to your hand is the eye relief. You can measure this distance with a ruler, or you can make a cardboard tool that includes both a flat surface for the image of the exit pupil and a short ruler. Measured eye relief may not be the same as the scope maker advertises. There can be some manufacturing tolerances, but when the scope only has 2.75" and the spec sheet says 3.5" it would be enough to make a real disgruntled .300 Magnum shooter.

FIELD of VIEW (FOV)
The side-to-side measurement of the circular viewing field or subject area. It is defined by the width in feet or meters of the area visible at 100 yards or meters. A wide field of view makes it easier to spot game and track moving targets. Generally, the higher the magnification, the narrower the field of view.

THE OPTICAL TRIANGLE
With scopes, there is an “optical triangle” that comes into play consisting of Magnification, Field of View, and Eye Relief. When you increase or reduce one side of the triangle, it affects the other sides. That is, increased magnification reduces eye relief and FOV; increased FOV at a given magnification reduces eye relief. Increased eye relief reduces FOV at a given magnification etc. Magnification is the biggest factor in FOV. Many scope brands tend to be slanted to one side of the FOV/eye relief triangle. This is good because it allows the consumer to pick which is most important for the particular application. A bigger FOV may help you get on target quicker, while longer eye relief may be necessary with a harder recoiling rifle. A larger diameter ocular lens also contributes to a larger FOV.

OCULAR LENS
The lens closest to your eye or Eyepiece. This part controls both FOV, as mentioned above, and focus. On many scopes the ocular housing can be turned to focus your scope for your eyes. Most are focused so they look sharp at the close distances you look through in the store when comparing and buying! This usually needs to be changed for the real world. There is a narrow lock ring at the junction of the ocular housing (or eyepiece) and the scope body, just behind the power ring of a variable – if there is no lock ring, the scope is an eyepiece focus ring type.
1: Loosen the lock ring (if it has one) and rotate the eyepiece counterclockwise five times. (Warning: Do NOT turn the eyepiece more than 15 rotations counterclockwise or this can break the waterproof seal)
2: Look through the scope toward the sky, or at a white wall about 10’ away. Rotate the eyepiece, or eyepiece focus ring clockwise until the reticle appears sharp and black at a quick glance. Do NOT look through the scope as you turn the eyepiece, as your eye will adjust to the out-of-focus condition. Glancing through the scope will immediately reveal the reticle as distinctive and black when it is properly focused.
3: Lock the eyepiece by tightening the lock ring. Some models (Like the Burris FFII) have an eyepiece focus ring and no lock ring is provided or necessary. Forgetting to lock the ring can result in inaccuracy when shooting as well.

RETICLES
“Reticle” is a fancy name for the crosshair or any other aiming point (post, dot, etc.) For years the most popular scope reticles were the crosshair and the post-and-crosshair. European scopes, including Russian sniper types, often favored a post setup of some type. In 1962 Leupold introduced the Duplex, a crosshair with heavy outer sections and slim lines in the center. This makes sense. The thick shank grabs your eye, directing it to the middle of the scope, where the thin intersection allows you to aim precisely. In addition, knowing how the wire subtends at a set magnification and yardage, you can use the Duplex as a rangefinder. This “plex” style is now the overwhelming choice of most scopes today. Variations include a heavy plex that is superior in low light, and a plex with small unobtrusive marks below the intersection that are used as aiming points or even rangefinding points, at longer range.

The more complicated military type is the “Mil-Dot” oval dot - while the civilian hunting version, a series of small lines, was popularized by Burris recently in the FF II line as the “Ballistic Plex”. Other scope makers offer a similar plex but use another term such as Leupold's "Boone & Crockett" or Nikon's "BDC" reticles. This is a very nice bonus and becoming deservedly popular. A ballistic chart, in the form of a sticker, matching a given caliber is provided, or you can do your own experimentation and make your own chart. The Ballistic Plex type requires your rifle to be sighted in at either 100 or 200 yards depending on your cartridge and bullet selection. You set your scope on (usually) the HIGHEST magnification and either used the supplied stickers/data, or make your own by actual firing at each 100 yard increment for your own very precise table.

Examples of new-style multi-reticle systems:
Mil-Dot reticle:
Image

Burris Ballistic Plex reticle [size=8pt](Burris also offers other variations - more on this later)[/size]
Image

Nikon BDC 600 reticle
Image

TIP: Shooters must remember that in our common Second/Rear focal plane scopes (more about this below) that these ranging marks like the Boone & Crockett are only accurate when the scope is set at the correct power (should be in your manual, but you can email or call their techs). In other words, if the scope is turned down to its lowest power, you shoot over the top of whatever you're aiming at short of a barn.

The Mil-Dot system is very good, but requires quite a bit of learning to really make proper use of its milliradian. Based on a 360 degree circle system, a milliradian is about 1/17th of an inch. You divide the target’s height in mils by the number of interstices subtending the target to get the range. Well, you get the picture…

The plex isn’t the beginning and end of reticles. There are many varying types, such as the post, the dot, the cross-hair-and-dot, etc. Most of these are for more specialized uses, and some are generic to the scope/type/country of origin. Russian sniper scopes, for example, have there own type of reticle and there is no option for others. There are also illuminated reticles for use in poor light conditions. Most rely on a battery of some type and brightness can often be adjusted.

Mechanical reticles are made out of something like flattened platinum wire or ribbon wire suspended on a mount. Another way to make a reticle is to use a photo-etching process on metal foil. Proper tension of this type is very critical because a slight error either way can result in failure under recoil or failure with extreme temperature variation. Pointing this type at the sun can also burn them out. This type is done offshore because of the nasty chemicals used during etching. The very thinnest crosshairs are fashioned from spider web!

The reticle can be placed in two different positions in the scope:
First or front focal plane reticle placement is common on European scopes. These scopes have the reticle placed in the objective end, or forward of the erector system. This causes the reticle to be magnified at the same rate as the target. When you increase the magnification the reticle will continue to cover the same amount of the target as it did at low magnification.
One advantage of having the reticle in the first focal plane is with a multi-point mil-type reticle because it stays the same size relative to the target.
Most American made scopes (including those made for U.S. companies in the Far East), have the reticle placed in the second, or rear focal plane. In this position, the reticle will not be magnified at the same rate as the target. In other words, when you increase the magnification on a rear focal plane scope, the reticle will subtend less of the target than it does at low magnification. For the majority of hunting applications in the US, positioning the reticle in the 2nd focal plane is desirable. The biggest drawback to our common second focal plane scopes is that on variables, they are susceptible to POI shift as the power is zoomed. Careful construction helps eliminate this, but it is not an issue with first focal plane scopes as that part doesn't move when zooming.
Most European scopes have the reticle in the first focal plane and the reticle will remain constant, however quite large on high magnification. For most American hunting applications it is too large. Most European hunting is not done at such long distances.
Many tactical groups prefer front focal plane designs because common tactical reticles serve a dual purpose: a point of aim and a means of measurement. Reticles such as a mil dot are based on a specific subtension and require exact feature spacing to be accurate; if this type of reticle is used in a rear focal plane design, the scope must be used on a single, specific magnification (typically high power). Placing this type of reticle in a front focal plane design allows the operator to use the scope on any magnification while retaining the exact spacing of the reticle features.

If the reticle cell has shifted, due to time and recoil, it will create excessive parallax, meaning the reticle apparently wanders around the aiming point when you shift your head behind the scope. Groups tend to string up and down when this happens. The reticle cell can also come loose. I have had this happen to the reticle on a cheap Chinese SKS scope which turned the reticle into an “X” after a minor amount of use. You can’t hit anything when this happens either!

PARALLAX
A condition that occurs when the image of the target is not focused precisely on the reticle plane. Parallax is visible as an apparent movement between the reticle and the target when the shooter moves his eye anyplace but dead-center behind the scope's field-of-view - or, in extreme cases, as an out-of-focus image.
Most center-fire riflescopes under 11x are factory-set parallax-free at 100 yards; rim-fire and shotgun scopes at 50 yards. Any scopes over 10x requires some sort of method to eliminate parallax. These scopes usually have a special range focus (adjustable objective or AO) to adjust for parallax. Newer and tactical versions often feature a side-adjustment, normally placed on the left side of the scope opposite the horizontal turret adjustment. Parallax is really apparent when you try to focus on a target with a standard rifle scope at 25 yards. The crosshairs seem to “float” around as you slightly move your head/eye behind the scope.
As mentioned in the section above - if the reticle cell has shifted, due to time and recoil, it will create excessive parallax, meaning the reticle apparently wanders around the aiming point when you shift your head behind the scope. Groups tend to string up and down when this happens.

POINT OF IMPACT (POI) / POINT OF AIM (POA)
The spot where the bullet actually hits the target is the Point Of Impact. The place on the target where the crosshairs are placed is the Point Of Aim. You adjust both Windage (the horizontal left-to- right movement) and Elevation (the vertical up-and-down movement) to Zero the riflescope at a certain distance. These is usually done by moving the adjustments a certain amount of “clicks”, which then moves the POA to coincide with the POI.

SCOPE CONSTRUCTION
Construction will make keep that new scope reliable and working right way down the road. It will make sighting-in easier and lessen the chances of the “big one” getting away because of scope failure. It will hold up when recoil levels equal or exceed that of the .30-06 for years. It will keep you from fogging up. It will keep your zero – zeroed after the normal knocks of carry and use. It will save you from buying new scopes down the line and going through the re-mounting and re-zeroing all over again.

General scope parts [sub](Bushnell)[/sub]
Image
Detailed scope parts and image path [sub](SWFA)[/sub]
[img width=600 height=185]http://www.samplelist.com/pictures/details.jpg[/img]

TUBE
Tubes with thicker walls and less joints resist stress forces that come with mounting and knocks. One piece main tubes are best here. As mentioned, there are both the U.S. standard one inch (25.4mm) tubes and the European-derived 30mm tubes. The main advantage to the 30mm tube is that is has more available windage and elevation adjustment range for extreme cases.

NITROGEN GAS
The scope is filled with dry nitrogen gas to eliminate moisture and fogging. Quality of the seals used here is very important, because nowadays, most scopes are nitrogen-filled, but can leak due to poor seals. Variable scopes with their moving power ring are harder to seal than fixed power scopes. Quality sealing with longevity in mind adds some cost.
This is why cheap fixed power scopes may have much less of a tendency to fog compared to their inexpensive variable power brethren. A scope contracted to be built to spec for a scope “maker” in one of the two Japanese optic factories has 8 levels of waterproofing to choose from. Target price of the intended product drives the choice of “how good”. Something to keep in mind anyway.

The newest technology in this area is to use Argon gas, or an Argon/Krypton gas blend rather than dry nitrogen gas. Claims are that Argon gas does not absorb water or react chemically with water. It maintains its protective properties over a wider range of temperatures. And lastly, Argon gas does not diffuse as quickly as other elements, extending the service life of the optics longer. Of course you might have to pay a bit more for this new technology at the present time. For example, Leupold offers Argon/Krypton gas in it's more expensive VX-3 line of scopes.

ERECTOR TUBE
The interior lens assembly that is actually moved when adjusting the windage and elevation is held in place by springs. The strength of these springs determine how well a scope may stay in adjustment or “zero.” Some scopes may even feature double springs, and Burris has gone to the expense on some of their scope to incorporate a “Posi-Lock” feature that solidly locks in your adjustments with a retractable steel post. This keeps a scope from failing when subjected to heavy recoil.

These springs can go bad. If the spring that controls vertical adjustment has gone bad, for example, the groups will string very vertically, sometimes a foot or more, with no sequential pattern.

SIGHT ADJUSTMENTS
Quality scopes may use steel-on-steel adjustments that are more positive and repeatable click adjustment. Cheaper ones use plastic here. While quality “name” scopes usually use good stuff, there can be issues with plastic, depending on grade and cost, either right off the bat, or down the road a few years with adjustment repeatability. Higher end scopes like the Nikon Monarch series use plastic, but repeatability seems to be fine for normal hunting use. The budget “Mart” loss-leader scopes would be suspect. Burris and Leupold VXIIs (and up) use steel-on-steel, to name a couple. Some scopes still use a friction non-click adjustment. If done well, this can work fine for normal hunting sight-in. Leupold’s older Vari-X II and new VXI and Rifleman line use this method successfully although a click system definitely spoils you.

Another factor to consider here is the amount of clicks per inch in the adjustment. Some just give you 1 click to move the POA ½” at 100 yards. Others may give you 1/3” per click. Very common is the ¼” per click – which translates to 4 clicks will move the POA 1” at 100 yards. Some have 1/8” clicks, which is almost too fine for most uses. Some scopes will give you a “witness mark” ring of numbers on the outside of the adjustment that can be turned to put the “0” at your final zero adjustment. Then if the need arises, and you have to adjust again, you can easily return to your original setting (marked 0). A nice, but not necessary feature.

One thing to be aware of, is that sometimes the adjustment value of each click doesn't match the claimed adjustment value. This can even extend to the vertical click being a different real-world value than the windage click. For example, a recent test of the nice Bushnell 6500 Elite 4.5-30x50 showed the value of the elevation clicks to be right on specified 0.25", but the windage value was actually 0.035". Another brand scope's actual click value was 0.28" rather than the specified 1/4". Even if they are off the specified value, it really isn't a big deal as long as they are consistent and you know the actual value - especially if you use the adjustment to crank on extra elevation when shooting at different yardages. You should probably check this out on paper first, rather than merely taking the manufacturer's word for it.

Reticles come from the factory set in the center of the adjustment range. Usually the further from the center you adjust them from center, the more erratic or "off" the adjustment value becomes. A 1/4 MOA click may not always adjust as advertised. Fire a shot - the impact may be an inch high. Turn the turret dial 4 clicks and fire another shot. Impacted 1 inch low, and so on. This can be the frustration you get when the reticle is close to the edge of the adjustment range. So you should make a serious effort to mount the scope, shimming if needed, or using something like the Burris Posi-Align rings.

TIP: Gun scribe John Barsness says that the erector tube lags somewhat behind a retreating adjustment screw. After a major adjustment, sometimes the erector tube even slides a little to one side. This is why many rifles often don’t shoot exactly to where they should after a major adjustment: The erector tube hasn’t settled into its final resting place. Over-adjusting is the easiest way to settle the erector tube, far cheaper than firing a “settling shot” and much more reliable than tapping on the turrets. Just go a few clicks past the theoretically correct adjustment, then turn back those few clicks. It doesn’t matter if you over-adjust two or three or four clicks, just make the number consistent.

Gun writer Norman E. Johnson says to always complete the windage and elevation adjustments with a CLOCKWISE screw thrust. This pushes the screw firmly against the erector tube, as opposed to the return springs doing this function. You will always arrive at a more precision adjustment in this way as you obviate the effects of a sluggish leaf spring thrust. If you want to go up or right with the reticle, simply go past your desired setting and turn the dial clockwise for final adjustment. This maneuver works with nearly all scopes regardless of price or quality.
This is the surest way to have the adjustments end up right where you want them!

MAGNIFICATION
The most personal factor in the choice of a scope is the magnification, or magnification range in the case of a variable power scope. Way back when, snipers used 3.5x scopes, hunters used 2.5x in the woods and 4x for open country. Now the variable power 3-9x is king and shooters are increasingly getting scopes of ever higher power, usually in some type of variable outfit, which seems to reflect the American idea of “more must be better.”

While magnification may bring the object closer, the additional power magnifies and flattens the environment as well as the atmosphere between the viewer and the object being viewed. Air density and mirage near the surface of the earth are greatly enhanced, destroying the fine resolution and contrast of which the scope is capable. The detail seen (and for binoculars and spotting scopes, the fine focus available) at a lower power, quite often render a much better understanding of the object viewed.

The advantage to the lower power scope is a wider field-of-view. This can be critical in the woods, or with running game. They also tend to be very bright. When the target is stationary and a longer ways away, the higher Xs are nice to have. Drawbacks include a much smaller FOV, and less brightness at higher settings. Secondly, running game is harder to hit and the extra magnification amplifies your unsteadiness – sometimes to a point where it is unnerving! (You always have that unsteadiness, you just can’t see it!) The 3-9x covers most of the range that most shooters and hunters use. Because of the huge number sold, they are usually cheaper than any other variable power range. However, if your shots are up close, you might consider trading a few Xs off the top for a much larger FOV in a 2-7x. An example of this comes from the Burris specification sheet and is representative of other brands as well. Their standard 3-9x has a FOV of 33 feet at 100 yards at 3x. Going down just one power notch, the 2-7x gives you a third bigger FOV, or 45 feet at 100 yards at 2x. This could be very critical in close woods or running game shooting and worth the extra cost over a standard 3-9x. Lastly, on a hunting rig, the lower power variables are lighter and smaller on a rifle you may carry all day. So give this stuff some thought before buying.

Scopes in the 4-12x range, and above, are really slanted either to dual purpose varmint/big game use, or varmint/target use. The newer 4.5-14x range is popular for varminting because, over 14 power, mirage often makes it look like you are look through a swimming pool at the target. Consider that even 7 power makes a deer at 700 yards look as if it was only 100 yards away. Consider that you begin to give up wide FOV on the bottom end. How many can’t see a deer at 100 yards well enough to shoot?
Snipers have also gone the higher magnification route, with a simple, reliable, fixed 10x being popular. Variables are making large inroads here, due to their increasing reliability and the ability to either go higher or lower for specific shooting situations.
As previously mentioned, magnification is very personal, just think through the intended usage and look at some of the other factors as well before making a final decision.

Zoom or variable magnification scopes have become so universal that decent fixed power scopes are becoming rare, despite their advantages of better anti-fogging, simplicity, lighter weight and smaller size. They have become a specialty or nostalgic item. One place they really stand out nowadays is on a .22 rimfire. A four power scope still offers everything needed on one of those rifles.

Drawbacks to variable zoom magnification scope would include, a larger scope, more weight, more chance for waterproofing to fail, the possibility that you may inadvertently have it on the wrong (usually too high) power setting when a close or running opportunity presents itself. This human-based problem can be cured with discipline, but it is an entry point for Murphy’s Law!
Many scopes have a shorter amount of eye relief on the higher power settings, as mentioned in the Eye Relief section. You could place one on your .300 magnum and end up "scoping" yourself at 10x. This is mostly an issue of scope placement and picking a scope with enough eye relief on high settings to begin with.
Due to the placement of the reticle in the second focal plane, most zooms are also often prone to change POI when changing the power setting. More expensive scopes go to great lengths to minimize or eliminate this tendency in construction and European and some other scopes place the reticle in the first focal plane to eliminate this (albeit with the thickening of the reticle when power is increased.) Less expensive scopes make it more of a crap shoot sometimes, so a fixed power is always the very safest bet with them.

TIP: When changing power on a variable, a 3-9x for an example, start at the bottom power, 3x and always go up to the desired power, say 5x. When changing back to a lower power, go all the way to the top (9x), then down to the bottom (3x), and then back up to the next desired power (e.g. 6x). If you want to stay on the bottom (3x) it is best to go to that power and add a fraction of a turn upward. This is all it takes for most zoom systems to track properly. This minimizes POI zoom shift with decent scopes. It works.

HUNTERS WHO USE HIGH MAGNIFICATION SCOPES TO IDENTIFY TARGETS, OBJECTS AND PEOPLE ARE DANGEROUS. GET A BINOCULAR.

WEIGHT
Is it a factor? The more power, and thus more lenses and bigger objectives may make a scope that unbalances your hunting rifle, making it rather top heavy. This is less of an issue with a heavier target or varmint rifle. On very heavy recoiling big bore rifles, a heavy scope has a lot of inertia due to the weight, and can have more problems with recoil failure than a quality lighter scope. If you are building a lightweight “Mountain” hunting rifle, a lighter scope can shave off more ounces less expensively than any other method.

SCOPE BRIGHTNESS AND LIGHT GATHERING

LENS SIZE
It’s not all about the size of the objective lens. Riflescopes with monster objectives may actually transmit less light than much smaller instruments.
Yes, the big front window does let light in, but it’s the size of the exit window that determines how much of that light gets into our eyes.
The quality and quantity of anti-reflection coatings enhance or degrade brightness too.

How does it work?
The objective lens lets light/image in.
Several internal lenses pass that image on to the eyepiece where it is additionally magnified and passed out to our eyes.
The higher the magnification, the smaller the exit window, commonly called Exit Pupil (EP.)
The scope’s EP corresponds to our pupils, which dilate to a maximum of 7mm in a young person, declining to about 5mm by age 50 and another millimeter each decade after that. If a scope’s EP exceeds the diameter of ours, the rim of extra light spills out onto our irises, wasted.

You can see the EP of any optic by holding it at arm’s length toward a bright area and looking at the eyepiece lens. That little bright circle is the EP. Through it pass all the waves of light making up the image of that deer you’re hoping to shoot. If you have a variable power scope, turn the power ring and watch the EP change diameter. Reduce power and the EP enlarges. Crank power up and it decreases.
EP is calculated by dividing objective lens diameter by magnification.
Thus, a 40mm scope at 4x gives a huge 10mm EP. Cranked up to 10x, the instrument still delivers a 4mm exit pupil. A 4mm EP provides more than sufficient brightness for targeting a deer at one-half hour after sunset on a cloudy day in the woods (or legal shooting hours most places). An optic with an exit pupil of 1.3mm restricts light transmission to such a degree the optic's effectiveness is rendered almost useless.

As an illustration of how all factors optical are co-dependent, magnification and image quality come together at a specific exit pupil diameter regardless of the much advertised twilight factor. When a combination of objective lens and magnification dips below 2.5mm, image quality suffers substantially. Above 2.5mm, image quality becomes more apparent, as each increase in exit pupil diameter enhances resolution and contrast.

LENS COATINGS
Anti-reflection coatings contribute to brightness by managing the light efficiently.
Raw optical glass reflects about 4 to 5 percent of the light that strikes it and as much that passes out of it. The loss occurs at each air-to-glass surface, so a scope with seven lenses could lose 70% of the light. Adding insult to injury, the reflected light bounces willy-nilly, lens to lens within the scope, appearing as a foggy haze that reduces contrast and obscures the principal image.

Way back when, optical engineers at Zeiss discovered that coating lens surfaces with exotic coatings could reduce reflected and air-glass losses. Eventually they found that multiple coats of different exotic compounds really helped out. Thus the coated and multicoated lens was born.

Definitions to learn when shopping for scopes:
Coated lens – A single layer on at least one lens.
Fully-Coated – A single layer on all air-to-glass surfaces.
Multi-Coated – Multiple layers on at least one lens and all surfaces are coated at least once.
Fully Multi-Coated – Multiple layers on all air-to-glass surfaces. This gives maximum performance/brightness. This is the best.

These anti-reflection coatings reduce reflection loss to as low as .2% per surface (claimed by manufacturers anyway.) Let’s be generous and give them .5% at each surface. That totals 7% reflection loss in the above hypothetical 7-lens scope for total light transmission of 93%. Whether the manufacturer’s numbers are correct or not, we do know that multiple layers of these coatings, preferably on every lens surface, are our best defense against reflection loss and internal flare and glare. This is good news because, although multi-coatings do add cost, they don’t add bulk or weight or require unusually high rings. It’s almost an optical free lunch.

One exterior lens coating that offers both optical improvement and and all-important lens protection for those highly refined lens surfaces and exotic coatings is Leupold's DiamondCoat 2. This is an ion-assist lens coating, for higher light transmission and the greatest level of abrasion resistance Leupold has ever offered. DiamondCoat exceeds military standards for hardness and durability. Doubtlessly, in the competitive scope and tactical scope market, other makers are coming up with similar coatings for their more expensive models.

There are also lens coatings that do not contribute to the optical quality of the scope, but rather aid other concerns. The first of these is a hydrophobic coating. This is a exterior lens coating that repels water. Bushnell introduced this coating technology and calls it "Rainguard". Raingard doesn't prevent all moisture from sticking to the lens, but does repel enough to provide an image usable for most shooting.

RESOLUTION
Another contributor to a bright scope is resolution. Resolution is one of two premier requirements of most optics. We see this as a crisp, sharp image that jumps from the background. It is created primarily by lens quality, a precise grind of clean glass that focuses a perfectly sharp image without significant aberrations in lines and colors. Remember that the better glass costs money. Above 10x, chromatic aberrations begin to soften edges, because not all colors in the spectrum can easily be focused back to the same point. The result is sort of a double image , one laid over another that is slightly larger. The fringe color is usually yellow or green. Apochromatic, HD and ED lenses largely correct this color fringing. It also takes adequate transmission of light to achieve good resolution.

CONTRAST
Contrast plays a role here too. It is the second premier requirement of most optics along with resolution. Contrast is the apparent difference in brightness and tone between a tan antler time and a brown tree limb. Anti-reflection coatings enhance contrast by minimizing glare that mutes contrast. Lens quality, its ability to efficiently pass certain wavelengths of light (especially in the blue spectrum where human eyes are least sensitive, i.e., during twilight) plays a big role in boosting contrast too. A scope that transmits an extremely sharp, high-contrast image can look noticeably brighter than another that actually passes more light, but poorly resolved.

PLASTIC LENSES
As mention earlier, good glass is expensive. There are only a handful of companies that make the glass to buyer specifications for virtually all the optics companies. Yes, even our U.S. firms use Japanese glass. As a lower cost alternative, some outfits are using plastic lenses. It has the virtue of being lighter and cheaper, however, it is also softer, so it cannot be used for an outside lens surface that might get scratched. It also tends to scatter light rays and has poor color correction, scattering colors. Plastic can be molded into aspherical shapes that can produce a wider FOV and flatter images at the fringe. This is what Simmons does with its AETEC series. The lens is internal in that application. You can probably safely assume that the least expensive scopes make the greatest use of plastic lenses.

SCOPE SUMMARY
How much any of this matters is for each shooter to determine. At some point, improvements in optical quality become difficult to detect. Not all eyes are sensitive enough to see subtle differences, and if you can’t see it, why pay for it? And how much of this is really necessary anyway? Money being no object, we’d all buy the best. But when money is an object, we must compromise. Noted gun writer and columnist on optics Wayne Van Zwoll says that he begins that compromise by "recognizing that a scope is a glorified front sight. As long as it keeps its reticle where the barrel prints bullets, shot after shot, season after season, I can live with slightly less optical quality than required to observe the moons of Neptune." I would second that opinion.
If you are a hunter, you might want to reserve the big optical cash for high quality top-line binoculars and/or spotting scopes first. That’s what finds the game. A decent scope with as much of the above qualities, plus reliability, rain or shine, year in and year out, even with 300 Magnum recoil, can easily be found in the $200 to $400 range. Buying one for less than that requires a careful look into construction, coatings, quality of glass, repeatability of adjustments, etc. There will be more compromising on the lower end of the cost scale. Think carefully about intended usage and magnification.

What about Tactical scopes? Patrick Sweeney of the Front Sight Firearms Training Institute and gunwriter as well, has what he calls his "Good-Enough" law: "Yes, the military buys $1,200 scopes for rifles. Heck, they buy $12,000 scopes. Why? Because it costs more to helicopter the SpecOps team to their ambush site than you'll spend all year on guns, ammo, gear and flavored coffee beverages. The cost of a scope is nothing to the military; it's less than the HV/AC budget for the Pentagon building. If you have enough money that the cost of a scope is nothing to you, have at it. Otherwise, a $400 scope and $800 in practice ammo will serve you better in the long run."

Many of the practical among us want a do-it-all rifle and scope, a "universal" scope if you will. Inconvenience is the downside to using "universal" tools. It isn't always pleasant putting up with oversize equipment for small jobs in order to have adequate equipment for big jobs. But 3-9x and 4-12x zooms come fairly close. That is why 3-9x is the biggest seller (and thus cheaper).
And lastly, remember that those big 50mm “Light Gathering” objective lenses are basically hype, require extra high rings, degrade proper cheek weld, and make the scoped rifle top-heavy and ungainly. So beware!


SPRING-PISTON AIR RIFLE SCOPES
Powerful spring piston airguns can tear apart an ordinary rifle scope.

To understand why this happens, we need to examine some basics about recoil and spring-piston airguns. First of all, it’s hard to imagine that a spring-piston type airgun would have any recoil at all. As we all know, with normal firearms recoil is generated by the bullet and gases exiting the muzzle. As you increase the weight and speed of the bullet and the speed and volume of the gases, recoil will increase. The only thing mitigating the recoil generated is the weight of the firearm. Heavier guns recoil less than light guns all other things being equal. All recoil is generated to the rear.

On an airgun, a typical pellet will weigh only a minuscule 7.9 grains, and velocities are usually well below 900 fps on most rifles, and less than 500 fps on pistols. Spring-piston rifles and pistols are complicated, heavy mechanisms and consequently often weigh as much as a normal firearm, or more. Therefore you could reasonable expect the recoil generated in them should be absolutely negligible. With that in mind, how was it possible then for a first-quality rifle scope that will hold up to the recoil of a .300 Magnum to be ripped apart by a spring-piston air rifle that doesn't kick at all?

Unlike regular firearms, spring-piston recoil is NOT generated by the pellet and the air being expelled out of the muzzle. It’s actually being generated by the piston and spring, both of which are very heavy components, especially the large steel spring. When the sear is released, the highly compressed heavy spring will jump forward with tremendous force pushing the piston ahead of it. As the spring and piston are moving forward, the gun is recoiling back against your shoulder with significant pressure. Now here’s the part that many people don’t understand. As the piston comes to the end of the compression chamber it will actually strike the wall with substantial force, and the gun will now bounce forward, recoiling away from you. In addition, this action generates vibration in all directions. Even if your spring-air rifle is "recoiless" like an RWS 54, that only means that they do not cushion themselves on the shooter's shoulder but have recoil forces within the rifle transmitted to the scope, if not the shooter.

So the essential elements to be remembered here are that recoil from a spring-piston airgun is not light but actually fairly considerable, and that there are two recoil pulses in opposite directions i.e. one to the rear and one forward. For a scope to withstand the recoil pulse away from the shooter, it has to be constructed specifically for that task. One thing a spring air rifle scope must have is a reticle braced both front and back.

Many scopes fail after a limited number of shots, sometimes as few as 20. Mostly this shows up as a reticle sitting loose or pointing in the wrong direction in the sight picture. Sometimes the reticle adjustment mechanism fails. Sometimes the warnings come in a more subtle manner in that point of impact changes all the time for no apparent reason.
Therefore, with a scope designed for a normal firearm may, or may not, have the reticle braced front and back. It varies from brand to brand, even perhaps from model to model. It is less likely that the bargain-priced rifle scopes feature this. Therefore some standard scopes will hold up to a spring-air rifle without issue, while others will fail. Even some expensive firearm scopes may fail miserably on a spring-piston air rifle. One cannot use any blanket statement that includes "all" or "none" on this issue.

Because the problem is well known in airgunning circles, actual quality air gun scopes are made to deal with the unique demands of the spring piston air rifle. So it lessens the crapshoot factor. You should check the scope specifications and warranty provisions before buying.

Besides being constructed specifically to withstand a forward and rearward recoil pulse, what else distinguishes an airgun scope from a normal rifle scope? Actually the most important characteristic of an airgun scope these days is its ability to focus down to 10 meters parallax free. Ten meters is the standard distance that most airgun competitions are held. Most varmint shooting is done under 50 meters. So you absolutely need a close-focus scope that is parallax-free at these unusually close ranges.

In today’s world, the distinction between air gun scopes and rifle scopes has become not only blurred, but very jagged as well, depending on the manufacturer. As it turns out, many of the main stream scope manufacturers have substantially re-engineered their construction designs allowing at least some or even all of their rifle scopes to be used on a “springer” air gun. Leupold, Burris, and Bushnell (except for their bottom 2 economy models) say that all their scopes are safe for spring-piston airguns. Weaver, Simmons, BSA, and Tasco say that not all their models are safe for spring-air rifles. All of the manufacturers listed offer designated airgun scopes. However the kicker here is that just because a rifle scope can also be used on a spring-air rifle, that doesn't mean they are ideal. The problem is a lack of parallax adjustment down to 10 meters, which is so critical for accuracy in these basically short range rifles.

An air gun scope is therefore defined as one with special construction features to handle the double recoil of a spring-piston airgun - and which has also been designed for parallax free viewing at 10 meters.

So “what is the difference between a rifle scope and an air gun scope?” The answer is “It really all depends on who the manufacturer is.” In the case of Bushnell there is essentially no difference between the two types. In the case of Burris and Leupold, there’s no difference in construction, but there is in optics. In the case of other manufacturers like Weaver, and Simmons, there is a definite difference in both construction and optics. For BSA, it’s a mixed bag. Some of their rifle scopes are mechanically compatible with air gun shooting and some aren't. However, none of their rifle scopes are optically compatible with air gun shooting. With Tasco, two of their rifle scopes are both mechanically and optically compatible with air gun shooting. They are the 8 X 40 X 56 Tactical scope and the Mag 40 6 X24. Interestingly, this particular Mag 40 scope is sold in the U.S. as a rifle scope and is sold in Europe as an airgun scope. The other scopes in the Mag 40 line do not meet either of the requirements for air gun shooting. Other Taco rifle scopes do meet the mechanical requirements but not the optical while even still others will correct parallax at 10 meters but not meet the mechanical requirements. You figure it out. There are plenty of scopes made especially for spring-air guns, like RWS etc., but your best bet is to check with sources that sell or cater to air rifles on the internet or elsewhere.

Decide on the quality of air rifle scope you want, then buy one a little bit nicer than that. It’s better to get something a little nicer than to get something you will regret having. The old saying, "You get what you pay for" is especially relevant with spring-air rifle scopes and optics in general.

You should also use unusually sturdy rings and mounts because the recoil and vibrations can affect this area as well - as weird as this may seem. Mounts will loosen and the scope can work loose in the ring and move. One piece mounts are usually recommend for high powered spring air rifles that have a lot of recoil. A one piece mount is more sturdy and can withstand the intense recoil of magnum air rifles.

Post reprinted with explicit consent of Frisco Pete, et al. Copyright 2014, http://www.sksboards.com
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Frogman Ladue
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Re: SCOPE, OPTICS & BALLISTIC INFO & FAQs

Postby Frogman Ladue » 01 Jun 2014, 21:53

CHEAP SCOPES

condensed from a Optics column by by Ron Spomer Rifle Magazine Jan 08

Okay, so you can’t afford a Zeiss Diavara Z scope, or even a mid-range Leupold, Burris, or Nikon. Now what?
Some cheap scopes will make you cry, but some will put meat in the freezer and antlers on the barn door – year after year.

Cheap scopes get that way by sacrificing various expensive parts. Just which determines how rugged and durable the scope will be, how bright and clear it will be, how glare-free it will be and how accurately it will adjust?

Here are some shopping tips:

1. As a general rule, go with fixed-power scopes at the low price end. Variables have more moving parts and are thus more likely to malfunction and/or leak. Typically, inexpensive variables shift point of impact as the magnification dial is turned. If you’re sighted dead-on at 9x, you could be 2 or 4 inches off at 6x or 3x. There is a way to live with this. If you need the range of a 3-9x, memorize where it hits at both extremes and leave it locked/sighted there for specific hunts [or situations].

While deer hunting in heavy cover, set it at 3x, adjust the windage and elevation dials to sight it in, and leave it at 3x (or 5x or whatever power you think best). When you go varmint hunting, set it to 9x, crank the necessary adjustments into the dials. And leave it at 9x for the duration of the hunt. It’s like using a different fixed-power scope for each hunt without having to remount – just re-sight. Each time you turn the magnification ring, the cam and slot wear slightly, which is why point of aim shifts over time. So don’t twist the power ring more than you have to.

2. Mount inexpensive scopes on low recoiling guns. Manufacturers use the cheapest materials they can get away with to create a scope designed to last roughly 1,000 to 1,500 rounds from an “average” deer rifle, i.e., no more recoil than a .30-06. The average hunter puts fewer than 20 rounds through such rifles annually. By the time his 30- to 60-year hunting career is over, he’ll have either traded or sold the scope or retired it, still intact. Mount such a scope on a hard-kicking slug gun or magnum, however, and its things inside could go “sproing” after two or three shots. There is a definite line between the recoil of lesser rounds and that of .300 Magnums and above. That is why scopes mounted on .30-06 and lesser rifles usually last a long time, but scopes mounted on a .300s start malfunctioning. The defects may be small, such as erratic adjustments or large, such as an adjustment spring coming loose, but a .300 Magnum (or .338 or .375) will find it, sometimes within one box of ammo. [Some of this can apply to the heavy bolt carrier and violent action of the SKS bolt slamming back and forth, transmitting shock to a bolt cover or other mount and then to a cheap scope. I have experienced this with a NcStar scope on a receiver cover mount on an SKS - FP]

3. If you insist on putting a cheap scope on a hard-kicker, make it a fixed power with a small objective lens. Less weight means less inertia stress and fewer parts to break.

4. Understand that low-priced scopes scrimp on lens coatings, and antireflective lens coatings are what increase light transmission (brightness) and decrease glare. Scopes with minimal antireflective coatings can work well on bright days if the direct rays of the sun are kept from shining on the objective lens. Expect considerable veiling haze if the sun shines on that front lens. A cheap scope is never going to look wonderfully bright and clear, but it should transmit images adequately for getting crosshairs on target.

5. Baby that baby. Expensive scopes are made more durable by milling them into single housings, so there are fewer joints to break or leak. Cheap scopes probably have front and rear tube halves screwed to the central turret structure. These are weak points liable to snap under stress. Don’t carry the gun by the scope. Don’t lean the scoped rifle on a car seat and have your kids sit on it.

Good Luck!

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MYTHS AND REALITIES OF OVERSIZED SCOPE OBJECTIVES

condensed from a Optics column by by Ron Spomer Rifle Magazine July 08

FACT: The higher the magnification, the smaller the exit window, commonly called Exit Pupil (EP.) The larger the objective (front) lens, usually the greater the EP. Divide objective diameter by magnification and the product is the EP diameter.
The scope’s EP corresponds to our pupils, which dilate to a maximum of 7mm in a young person, declining to about 5mm by age 50 and another millimeter each decade after that. If a scope’s EP exceeds the diameter of ours, the rim of extra light spills out onto our irises, wasted.


European scope makers began touting big objectives for enhanced brightness about 20 years ago. They were already building the big scopes for nocturnal boar hunting, a tradition in Europe. What they needed was a market in America.

The subsequent marketing campaign was brilliantly successful. True to their reputations, Americans went for "bigger is better." Simmons got in on the act marketing their "44 Mag" 44mm objective scope with tremendous success. Now 50mm is the trendy size for big objectives. Many Yank hunters today haul around scopes suitable for picking off vampires at 2 am in a New York sewer. Is that really necessary?

Maybe. Sometimes.

As advertised, big objectives do let more light into a scope. They don't magnify it or increase it. That requires battery power and the technology used in night vision binoculars. Glass optical instruments merely pass on whatever ambient light strikes them. Since glass always reflects some of the light striking it, the more light you let in, the more you get out eventually. Magnification reduces light output. The relationship between objective diameter and power is reflected in the diameter of the exit pupil (EP), the window in the eyepiece through which the final image emerges. That's the critical one. Divide objective diameter by magnification and the product is the EP diameter. So a 10x 50mm yields a 5mm EP, while a 10x 33mm produces a 3.3mm EP. At 6x the 33mm scope will have an EP of 5.5mm - bigger than the 50mm scope at 10x. And you want to know a secret? You don't need 10x to shoot a running whitetail at 200 yards. Or even a standing one at 400 yards.

In my long experience, a high-quality, fully multicoated scope with a 28mm to 42mm objective is bright enough to place black crosshairs on anything but a black animal 30 minutes after sunset on a cloudy evening, if the EP is at least 4mm. In most cases this will work at 40 minutes after sunset. The downside to such minimal EP is that it demands perfect form. If you align your eye slightly off-center with the scope, too far forward or too far back, you'll pick up that annoying edge blackout. A larger EP leaves more edge "window" for you to wander in before the dark edges come into your vision. So if you have trouble aligning head/eye perfectly behind your scope, you might want a large exit pupil, in which case the bigger the better.

You get plenty of EP room in a 33mm or even a 28mm objective scope by dialing down power. At 4x the 28mm will yield a 7mm EP. That's as wide as most [young] human pupils can dilate. But 4x does not reveal a distant animal as well as 10x. Even when high magnification compromises EP significantly, say down to 3mm, it shows greater detail simply because the object covers so much of your retina. It's just like walking closer to something in dim light in order to see it better. The light doesn't get any brighter, but it strikes such a higher percentage of your retina that it looks brighter. So, if you anticipate game at the edge of dark 300 yards and beyond, you might want 10x, and 10x looks a lot brighter coupled to a 50mm or even 56mm scope than a 40mm.

The drawbacks to big objective scopes...

1. Heavy. Most weigh 17 to 24 ounces... you might not appreciate carrying it up a mountain...

2. Bulky. The diameter means the entire scope will sit atop extra-tall mounts, so you may need to raise your head off the stock comb in order to see into the eyepiece. That slows down and compromises accuracy... A high scope will unbalance an average-weight rifle tending to pull it to the side and slowing down fast handling. The high scope will also catch on branches and be impossible to stuff into a traditional saddle scabbard. Many don't even fit standard soft cases. One exception to this is their use on AR-type rifles. The AR15 design not only is stocked to allow them, but the design allows a reasonable scope height with big 50mm+ objectives because the objective hangs off of the receiver over the somewhat low barrel position, and this gives a lot more room than on a typical bolt action.

3. Expensive. Glass costs. The bigger the glass, the bigger the price.

4. Less durable. During recoil, extra-heavy glass is more likely to pull loose from its moorings.

5. Rip-Offs. Huge objectives do little good if they and/or internal lenses aren't coated to minimize glare and maximize light transmission. Just because a Swarovski PV 3-12x 50mm transmits the most incredibly bright, sharp image you've ever seen doesn't mean a Cheapco XT 3-10x 50mm will match it. Some discount scope makers throw poorly ground glass onto scopes with single-layer coatings or no coatings at all. The result is so much internal glare that you can't pick out the target from the haze. To sharpen such poor lenses, some manufacturers place field stops (glorified washers) inside the objective bell behind the big objective lens. Like a small f-stop ring (diaphragm) in a camera lens, this blocks distorted light from the lens' edges. The image is sharpened, but also appears darkened. You pay for a 50mm, but get the light input of a much smaller lens (the size of the hole in the field stop).

Choose a 50mm to 56mm objective scope if you expect to do most of your shooting at high magnification near dark with minimal walking beforehand. That's where they really shine. And insist on fully-multicoated lenses throughout the instrument.

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SCOPE MOUNTING TIPS

Taken in part from the article "Advanced Scope Mounting" by John Barsness - American Rifleman magazine - December 2008

Definition - Farmer-Tight: When a screw is torqued right to the point of fastener yield, or so tight it's crushing what it's screwed to. A term derived from the process to prevent being two miles from the barn and having a few bolts work loose on farm equipment. See also: Good-n-tight; A quarter turn shy'a strippin; Just short of blew torque, etc.

Most everybody understands the basics of assembling the rings and bases and mounting the scope so that it has proper eye relief and the crosshairs are not canted etc; but there can be more issues that crop up from time to time. We will cover those issues first and end with scope placement.

OVER-TIGHTENING ISSUES

The sad truth is that you can take a brand new quality scope and either damage it, or make it malfunction just by how you mounted the scope on your rifle. Mounting a scope and rings on a rifle isn't rocket science, and it is a fairly easy task for most of us to do. However, there are some things to know that go against our normal way of thinking.

Most guys figure "MORE IS ALWAYS BETTER" but this attitude can lead to some SERIOUS PROBLEMS with scope mounting.
There has been a spate of complaints from various shooters. These range from "ring marks" on scopes, to expensive scopes that don't work right out of the box. Now anything made by humans can be defective, but an awful lot of the ring marks and bad scopes can be blamed on screwdriver crunching. An average hunting scope is a relatively thin aluminum tube with an "erector tube" inside, designed to be moved precisely inside the exterior tube. In variable scopes, the erector tube moves both side-to-side and lengthwise inside the erector tube. This is how scopes are adjusted for point-of-impact and magnification.

Now take the ring clamp commonly known as a scope-mount ring, and tighten the clamp until the exterior tube is semi-crushed. It's only natural that the scope fails to work the way it's supposed to, whereupon it's sent to the manufacturer because it was "defective right from the factory."

Some people damage there scopes because they have read that you need to ream scope rings. This process supposedly aligns and precisely sizes the insides of the mounted rings, allowing any scope to be mounted perfectly. When those people ream the rings with a special tool, the normally do so until the original insides of the rings are gone. Often the inside of the rings end up with nice sharp edges.
When they tighten the rings around their new scope so much metal has been reamed away that you have to really farmer-tighten the ring screws to hold the scope at all. The sharpened edges of his reamed rings bite right into the scope creating the unsightly ring marks.

I'd guess that the majority of the ring marks and scope problems encountered are due to over-tightening the rings.
[See the footnote at the bottom for more insight on the overtightening issue as applied in a non-scope ring context]

A modern scope is slightly flexible. In fact, the flexibility of the main tube is why we don't have to tighten the ring screws farmer tight. With just the right amount of tightening, the flexible tube presses against the rings and will stay in place despite considerable recoil. This is especially true with today's matte-finish scopes and matte-finish rings, but will also work with gloss rings and scopes mounted on a .416 Rem. Mag! The scope on the hard-kicking .416 stayed in place and didn't receive any ring marks.
The reason? Using only 20 inch-pounds of torque to tighten the ring screws.

Gary Turner of Talley [premium rings] recommends using 20 inch-pounds of torque to tighten Talley steel rings.
For the company's Lightweight rings, the aluminum model with integral bases, Turner recommends 15 to 17 inch-pounds.
D'Arcy Echols [an internationally known custom rifle firm in Logan UT] recommends tightening the 6-48 screws on their custom-made mount rings to 16 to 17 inch-pounds and the heavier 8-40 screws on the bases to 35 inch-pounds.
Garth Kendig, one of Leupold's technical guys, recommends 28 inch-pounds on Leupold's 8-40 ring screws and 18 inch-pounds on their 6-48 base screws [note the reversal of screw sizes between Echols and Leupold]
Warne recommends 25 inch-pounds for their Torx screws. In fact they sell their TW1 Torque Wrench this is a simple break-away design when 25 inch-pounds are reached - for use on all their rings and bases. If a person was to install very many scopes, this would be a good investment.

So we learn that you tighten:
6-48 screws to 16 to 18 inch-pounds
8-40 screws to 28 to 35 inch-pounds
Check your ring and base manufacturer's suggested torque specs if there is any doubt, or at least find out what size screw they are using for which part of the ring/base set. Some report slightly different torque values when they talk to Leupold tech support, but the main thing is not to get them farmer-tight.

How much force is 15 to 20 inch-pounds? I can loosen a Torx-head screw tightened to 15 inch-pounds while holding a typical screwdriver handle by the thumb and first two fingers of my [dominant - average] hand - therefore, using the same hold provides about the right amount of torque to the rings when installing them.
Another good tip is to use the factory supplied Allen or Torx driver that the mount manufacturer supplies - holding the short end in your hand and tighten as tight as you can with the ~1" lever and you will never have a problem with things loosening or getting it farmer-tight. Remember, just snug - definitely not as tight as you can get them with all the leverage you can muster.

Brownells offers a Magna-Tip Adjustable Torque Handle that can be set to any inch-pound rating from 10 to 70. A bit pricy for just one or two applications at $149.95 but some people may use it for other purposes as well. There are also other commercial torque wrenches available or perhaps one already in your tool box.

If slippage in a high-recoil gun might be, or has been, an issue M.L. (Mic) McPherson offers the following tips:
Make sure you thoroughly clean and degrease the scope tube and rings, then apply Loctite 609 to the ring/scope interface. Bonding strength of Loctite 609 exceeds 3,000 psi. Surface area of two 1-inch rings is about 3.24 square inches, so bonding strength - ignoring friction - will exceed 10,000 pounds, which should be adequate. Even a most onerous gun with a relatively heavy scope installed generates only about one-fifth this amount of recoil force between scope and rings.
He also recommends using Locktite 609 to bed your base installation. Wait for the bed to cure, then remove the base screws, clean and reinstall them with BLUE 242 Loctite and tighten. Then proceed to the ring installation.

LOCTITE® 609 Retaining Compound is designed for the bonding of cylindrical fitting parts. The product cures when confined in the absence of air between close fitting metal surfaces and prevents loosening and leakage from shock and vibration. It is available from: www.midwayusa.com/viewproduct/?productnumber=667024

OTHER MOUNTING TIPS

Aluminum or steel rings? When in doubt, go with steel. While aluminum can be as strong, steel is more forgiving and strips less easily. Cheap aluminum rings are not made from the highest quality aluminum alloys and should be avoided.

Weaver or Picatinny (U.S. Mil-STD-1913 "Picatinny")? Very similar but different. Weaver rings will fit in a Picatinny base, but Picatinny (which has thicker cross-bolts) will not normally fit in a Weaver base. You choice often depends on application. If you are scoping a hunting rifle then Weaver bases and rings are as good as anything. If your application is more military oriented, say scoping an AR15 or a rifle intended more for tactical use, then definitely go with Picatinny which offers more sighting options and all the military applications. Heavy duty tactical rings are almost always Picatinny.
Although Weaver rings fit a Picatinny base, free play is excessive (i.e. 0.060") so use Picatinny rings if you can, and once again, buy a good Tactical grade ring for extra strength.

Even in Picatinny mounting systems, there will be at least 0.010" of longitudinal (front-to-rear) play between the ring's cross-bolts and the mount rails. Once you have determined the correct scope placement for proper eye relief, put the scope/rings in their appropriate slots.
Before you tighten the ring-base nuts, move the rings FORWARD (i.e. towards the muzzle) to take up the play. This sets the cross-bolts against the rear of the mount rails, which is where the scope would eventually move under recoil. If, on the other hand, you had set the rings against the rear of the mount rails, that 0.010+" amount of play would get gradually shifted front to rear, likely changing the scope's point-of-aim and thus POI.

It is important to thoroughly degrease the screw threads and ring walls before mounting. Any degreaser such as Simple Green or brake cleaner, to name a couple of common ones, will work. Use Q-Tips for the screw holes.

In my opinion, another necessity to keeping your scope tight is to use 242 BLUE LOCTITE (or Lock-tite) on the screws. This will do more good than over-tightening the screws. A lot of our hunting rifles can see time spent in a pickup truck or on an ATV over the course of years of hunting. Screws can work loose through vibration. I personally have had the rear screw of the factory Ruger mount loosen and consequently missed a nice buck because the rifle shot 18" low at 100 yards. I have also had the Weaver base screws come loose on another rifle that spent some time in a pickup truck rack. So a drop of blue 242 Loctite is terrific insurance. I have never had any problems with getting the screws back out when I changed scopes out down the road.

ALIGNMENT ISSUES

Some scope mounting problems can indeed be traced to rings that don’t line up in the first place. However, this does not mean you should automatically ream every set of rings you install. Instead, invest in a set of scope alignment rods (also available from Brownells) which have sharp cones at one end of each rod. While installing a pair of scope mounts, the rods are mounted in the rings with the cones facing each other. If there’s any problems with scope misalignment, the tips of the cones won’t line up.
A little bit of misalignment is really no big deal. But if there’s serious misalignment of the mounting rod tips, say 0.10” or more, there are several solutions other than reaming. The easiest is to buy a set of Burris Signature rings. These are available in both the Weaver (Zee) and basic Redfield design, but with plastic inserts inside the rings forming a ball-joint around the scope. There’s no strain on the tube and it can’t be marked, though it might be semi-crushed if the rings are farmer-tightened. The inserts also come in different thicknesses, so that the scope can be closely aligned with the bore.

There is another trick in advanced scope mounting. While some (not all) modern scopes have wide-ranging adjustments, if the scope doesn’t point in the same direction as the barrel, odd things can happen. Aside from optical problems ranging from parallax to fuzziness, the click adjustments won’t be quite what the factory designed, because the adjustment screws will be off-center on the erector tube. So it really helps to have a scope align with the bore.

First, center the adjustments. Count how many revolutions each adjustment dial makes from one extreme to another, then set the dial in the middle of that range. Next, mount the scope and see if it points pretty much where the bore does. This is most easily done with a good collimator, but bore-sighting also works.

If you are not using Burris Signature rings and the centered scope is out of alignment with the bore, there are several solutions. One fine solution is to have custom mounts made. Talley will do this if you send them your rifle. Some mounts also have windage adjustments. Any Redfield-type ring has opposing screws on the base of the rear ring that will move the rear of the scope back and forth, and Conetrol and Gentry rings have opposing screws in both bases to move each end of the scope back and forth.

Burris Signature Zee Rings (henceforth abbreviated SZ) are a paradigm in scope mounts. They are harder and more fiddly to install but this is because of the wide range of latitude they have. SZ rings have floating inserts that ride in a spherical relief in the rings and act just like a ball-and-socket joint acts; therefore the rings do not have to line up!

Tightening the ring halves clamps the perfectly aligned insert halves onto the scope tube, regardless of misalignment between the rings.
Therefore SZ rings cannot transfer stress between the scope and receiver.
Eliminate any need to lap rings.
Allow easy and precise elevation bias, through the use of offset inserts.
Allow extreme elevation bias, though the use of a taller rear base.
Ease the chore of ideally mounting the scope, and;
Eliminate marring of the scope tube.
See the section on the bottom of this post for Burris SZ ring mounting how-to's from the pros.

There are scope bases available that add 20 or 30 minutes of bias for long range shooting. Sometimes application is limited, therefore with SZ rings and the appropriate offset, you can still add bias. You can also increase bias for super long range (or shooting calibers with a very arching trajectory like 45-70) in conjunction with a 20/30 minute biased mount.

If the misalignment is with elevation rather than windage and you choose not to use SZ rings or biased base mounts, the bases can be shimmed. You can buy specific shims from several places, including Brownells, but being cheap I generally cut them from beverage-can aluminum. With horizontally-split rings you can also shim in the inside of the ring with any number of things, including duct tape (one of my favorites).

Both shims, a 20 or 30 degree biased base, or Burris Signature rings can be used for a dedicated long-range rifle/scope setup where the elevation adjustments run of range too soon at very long ranges (600-1000 yards) if you have that problem with your current set-up.

POSITIONING ISSUES

Positioning is not usually a scope malfunction problem, but affects the user interface and dynamics of scope use. Scoped rifles should come up to the shoulder and align the scope with the eye as naturally as possible. Any time you spend moving your head around to find and center the scope FOV for your eye is negative. You should bring the rifle up and BE THERE.
This is dependent on both the scope and the mounts being just right. With most normal bolt-action hunting rifles, you are best served with a mount that allows you to mount the scope as low as possible. Modern hunting and tactical rifle bolt-action stocks are designed for scope use and will position your head (and cheek weld) properly on the stock. With stocks of normal objective bell size of 38-42mm, normally you would use medium height rings, the scope should clear the barrel and head position should be just fine. 50mm objectives can mess this relationship up a bit and normally require extra-high rings. Something to check for before you buy.

Older pre-1960 bolt-action rifles, lever guns, military surplus rifles, and any other gun that was designed for the use of iron sights likely will have a stock that is lower in the comb so you can see the irons better when you cheek down on the stock. Older scopes of that era usually were low-powered and had small objective lens of less than 32mm so a lower stock worked for them as well. You should attempt to mount your scope as low as physically possible on guns of this type, and would do well to mount scopes with small objective lens and power of 1.5 to 4x with under 32mm objectives to aid this quest. Sometimes with mil-surp rifles we can really never have an ideal set-up, but are forced to work with what we have and just live with it.

Depending on where the objective of your scope ends up as to front-and-back positioning may necessitate the removal of the factory rear sight on some guns so equipped. This seems to happen to me a lot. I carefully drift the rear sight out of its dovetail from left to right and put it in a zip-lock bag or little box with a note as to which rifle it came from if later installation is desired. Some remove the front sight as well in these circumstances if easily done, like with the Remington 700 front sight that is held on by two screws.

Proper front to back placement is critical for eye relief and quick target acquisition. You need to be able to see the full field-of-view in the scope, and do it when you mount the rifle without any adjustment. You also do not want to get “scoped” when a hard-recoiling rifle comes back with a scope mounted too close. So when you mount the scope, and before you tighten it all the way and can still slide it a bit, check to make sure it is good in this area. A good idea is to hold the rifle in all the positions you are likely to shoot the rifle in (standing, sitting etc.) and maybe even in the clothes you will wear (if the rifle is mainly used in cold weather) and experiment until you get the exact, proper eye relief when you bring the rifle up. Just standing there and looking through it the whole time will allow your eye and head to unconsciously adjust to the distance, so avoid that method.

Lastly, with any front-to-back placement - make sure the rear ring is a bit ahead of the power ring of a variable scope or the ocular bell of a fixed power scope. A minimum of 1/4" to 3/8" is a good idea to prevent any binding of the mechanism. For example, the otherwise-excellent Leupold fixed 6x 42mm scope is very susceptible to damage if there is not 3/8" clearance of the ring to the ocular bell.

The popular AR15 has its own issues with proper scope height. The A2 style is rather too high, while the A4 or flattop style is too low. There is not much you can do with the A2 carry handle except to mount one of those cantilever-type handle mounts which place the optic ahead of the handle and a bit lower. They can limit scope size and placement a bit as well.
The A4 flattop type is a good optics platform and was designed as such, however, as mentioned, the scope is too low for quick acquisition. You can use extra or ultra-high rings designed for such use, or you can use a scope riser base which mounts to the flattop rail and gives around 3/4 inch of rise and is used in conjunction with medium to extra-high rings depending on amount of rise needed. I find that rings that run around 3/4" in height give me the proper scope height when used in conjunction with such a riser base. You will have to check manufacturer’s specs to find the correct height as their nomenclature can vary. For example, Burris Xtreme Tactical rings list High as 0.75", Extra-High as 1.0" (for use with no riser base on AR15s). Warne Tactical rings list High as 0.605"and their Extra-High (which I use) as 0.770", with Ultra-High at 0.935" (again for use with no riser base on AR15s). So it pays to check beforehand. One good thing about the AR15 is, that due to its design, a 50mm objective lens has plenty of clearance using the same rings you would use with a standard 38-42mm scope objective. Height can be a personal thing, but if you can get with someone who can measure from the flattop to the center of your eye when you mount the AR, you will have a better idea of the height that is natural for you.

The last issue I see with scope mounting on an AR15 flattop is that with many scopes, and especially long varmint/tactical high power variables, is that the available mounting surface on the flattop rail or the basic riser base will not allow you to position the scope far enough ahead for proper eye relief. You snap the rifle up to your shoulder and have to pull your head back a bit to get the full FOV. Slow. Fortunately a couple of outfits make an extended riser base that extends over the barrel further to allow ring placement farther forward. So you get the benefits of a riser, and also much more ring and scope placement latitude. Rock River Arms sells what they call their RRA Scout Rail and Yankee Hill offers Rail Extensions in 5" and 6", to name a couple of this type.

To illustrate: I like to shoot jackrabbits where quick target acquisition is of paramount importance. When I first got an AR15 flattop, I tried an old scope on medium mounts on the flattop. It was a total failure as to acquisition. I then tried a combination high-rise-and-ring mount with built-in rings and a new Burris 3-9x FFII scope. Height was correct but eye relief was too close, causing me to pull my head back. It worked, but I felt it was not optimum. Lastly, I mated the RRA Scout Rail and Warne 0.770" Extra-High Tactical rings which gave me the same height as the one-piece scope riser/ring setup, and allowed me to mount the scope further ahead. My ease of getting on target and hit ratio went up dramatically with this small change. A bench shooter may not notice it because there are no time constraints, but in the field there is a dramatic difference. While each person has to engineer a set-up that fits their own physic and style, it pays to give it a lot of thought beforehand and not just pick up what is on the dealer shelf at the moment for cheap.

ABOVE ALL, THE NEXT TIME A NEW SCOPE STARTS ACTING WEIRD, REMEMBER THAT THE PROBLEM MAY NOT BE THE SCOPE, BUT THE NUT BEHIND THE SCREWDRIVER.

SEE ALSO www.charm.net/~kmarsh/mounts.html FOR MORE INFORMATION.

MOUNTING BURRIS SIGNATURE ZEE RINGS
M.L. (Mic) McPherson - Varmint Hunter #78

1. While you can just mount the base direct, M.L. (Mic) McPherson recommends that you bed the bases with 609 Loctite Retaining Compound. Wait for the bed to cure (approx. 3-6 hours), then remove the base screws, clean and reinstall them with BLUE 242 Loctite and tighten. You do this because you may want to get the base off easier again and the 609 loctite will really hold the little screws, making stripping possible. So you get rid of any 609 that has seeping into them and then use the blue stuff. No matter if you use 609 or not, make sure you use 242 on the base screws. Then proceed to the ring installation.

2. Install and tighten SZ rings and tighten attachment screws. Set the desired insert halves in bases, rest scope in inserts, place matching insert halves over scope tube (above ring bases), install ring tops, and install screws. Then tighten ring screws until the halves are uniformly gapped and the scope is just loose enough to freely move and rotate.

3. You might be wise to lubricate the outside of the inserts, to allow those to most easily move and freely rotate in the rings. I would use Sharp Shoot R case and die lube (this is a paste similar to Imperial Sizing Die Wax in texture - which would be a good alternate).

4. Position scope for needed eye relief. Rotate tube to align reticle to gun. Sequentially tighten ring screws uniformly, until those are tight enough for the application (as this post mentions, usually not anywhere nearly as tight as we might think).

5. M.L. McPherson marks the scope with a permanent marker at the front edge of the rear ring and then watches as he fires the first few shots. This mark makes it easy to tell if the scope is moving. If so, tighten the ring screws a bit more.

For the 1-inch SZ rings, Burris offers BIASED inserts in 5/1000", 10/1000", and 20/1000-inch offsets. With a 4-inch ring spacing, these inserts allow elevation biases from zero to about 36 minutes in 2 1/4-minute increments by using the correct combination of inserts to move the rear of the scope progressively higher.
With the use of progressively taller rear bases, biases up to about 2 degrees are simple.




****************************************************************************************************

TIGHTNESS ISSUES IN OTHER ARENAS - THE PROBLEM ELSEWHERE

Tom Houseworth is the crew chief for the Ben Spies' Yoshimura Suzuki GSX-R 1000 that won the 2008 AMA Superbike Championship. As such he is well aware of tightening issues in a high-RPM racing engine. Talking about torqueing connecting-rod cap bolts in particular, but with obvious application everywhere, he relates:
"Bolts are very stiff springs, preloaded by correct tightening to clamp the parts together. If you stretch them too little, they come loose. If you stretch them too much, the metal yields and, again, they fail by being too loose."
Cycle World - Feb 2009 - "The Last Superbike" pg.46

Post reprinted with explicit consent of Frisco Pete, et al. Copyright 2014, http://www.sksboards.com
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User avatar
Frogman Ladue
Posts: 460
Joined: 06 Apr 2014, 00:22
Location: USA, Ohio

Re: SCOPE, OPTICS & BALLISTIC INFO & FAQs

Postby Frogman Ladue » 01 Jun 2014, 21:56

SIGHT IN AND TRAJECTORY RELATIONSHIPS

When sighting in a rifle you should be aware of the relationship between the:
BORE LINE - which is a straight line from the bore of the rifle or pistol.
SIGHT LINE - which is a straight line from the scope, other optic, or iron sights.
BULLET TRAJECTORY - which a parabolic curve that the bullet path takes after it leaves the barrel.
ANGLE OF DEPARTURE - is the angle between the bore axis of the gun and the line of sight to the target.

Since the bullet starts to drop from level (barrel) to the earth from the moment it leaves the barrel - in order to have a zero sight-in at some reasonable point downrange we compensate by basically tipping the sight line down through adjustment until it intersects the bullet path at our desired zero yardage, whether it be 25 yards, 100 yards or 250 yards, it doesn't matter.

The following chart illustrates the relationship of all these concepts:
Image

Another thing to keep in mind that there is a CLOSE ZERO or POINT BLANK range and a distant or primary ZERO range due to the nature of the straight sight line intersecting the curving bullet trajectory path from it's start from the muzzle located below the sights. Between these two points the bullet path is actually above the line-of-sight. Generally we are trying to fine tune the more distant of the two zero ranges, because that is more accurate and is normally the range we say the gun is "ZEROED" or sighted in for. Another term could be PRIMARY ZERO range. The bullet path will drop below the line of sight after the furthest zero range where the sight line and bullet path cross, and as gravity and air resistance combined with a slowing of the bullet's velocity take a greater effect, the bullet path will drop radically until it strikes the earth.
To sum up or define, Point Blank or Close Zero is the first range that requires no elevation change either up or down to hit point of aim. the point where the projectile crosses the line that extends through the sights because of angular difference with the bore line. This first point will be just a few yards downrange even when the second Primary point is 100 yards or 300 yards.

The graph below illustrates how a .308 Winchester Federal Gold Match 168-gr Boat-tail Hollow-point load zeroed at a range of 250 yards leaves the barrel of the rifle 1.5 inches below the line-of-sight (a scope in this instance) and crosses the line-of-sight first, or "close zero" at 20 yards, then continues to rise above the line-of-sight until it drops back to coincide with the line-of-sight again at 250 yards or the distance we say the rifle is "zeroed" at.

TWO ZERO RANGE GRAPH - .308 Winchester Federal Gold Match 168-gr Boat-tail Hollow-point Match load
Image

Deciding on the proper zero depends on the cartridge your gun is chambered for and your specific needs. Some cartridges are considered shorter range rounds (like the .30-30 or 7.62x39mm) while there are very flat shooting Magnums on the other extreme.
Some people will never take a shot beyond 200 yards, while others may hunt or shoot at longer ranges. Some may want to stretch their shooting to the longest ranges they can. Rifle matches require set distance shooting that the rifle must be sighted in for. This all affects the zero decision.
A great many choose a compromise zero that keeps the bullet at a reasonable distance above and below the line-of-sight as far, or a little farther then they expect to shoot. By doing this the bullet will hit the target area, if not dead-center, at least in the target or kill zone. This is most popular among hunters who encounter game at varied and unknown distances, and works in a military combat setting as well.
Shooters who shoot at extreme ranges will often use a zero range that is a very distant yardage (500+), and one in which the bullet is a surprising height above the line of sight for quite a distance - in order to be on zero at the desired range, and not have too much drop below the line of sight at target ranges beyond the zero point. Sometimes scope mounts that are raised excessively in the rear need to be used to achieve this angle relationship at extreme range in order for the scope's internal adjustment not to run out of adjustment range. We will look at the differences in various zero ranges chosen from the three general areas we just looked at and their effect on bullet path next.

Some ballistic programs illustrate the bullet path. This ones you see here are from the Oehler Ballistic Explorer www.oehler-research.com/explore.html. The parameters can be set by the user and you can use the data to figure out what that bullet is doing after it leaves the barrel. The Oehler Ballistic Explorer allows up to 3 different comparisons at once, which is useful for illustrative purposes like on this post here. However, it is not free.
Of course you can use other ballistic programs as well. There are both free Internet programs and ones that you can buy available. I have listed some of the Internet ballistic program sites at the end of this post. If you have any favorites, please PM me with the details and I will get it added to the list.
In this particular illustration, the data is for the popular .308 Winchester Federal Gold Match 168-gr Boat-tail Hollow-point load (#GM308M). This is typical of a match or sniper load shot from one of the popular .308 Tactical rifles. It utilizes a Sierra 168-grain MatchKing bullet with a ballistic coefficient (BC) of 0.457.

These are some other factors that will affect your bullet path - and I've showed what data I plugged into the ballistic program to achieve my results:

Ballistic Coefficient is a scientific way of quantifying how streamlined a bullet is. The higher the number - the better the streamlining.

Velocity is 2600 feet-per-second. The more velocity, the flatter the trajectory. Chronographing the velocity of YOUR ammo in YOUR rifle gives the most accurate data.

Altitude is set for 2000 feet. Bullets shoot a little flatter the more altitude you have because of thinner air.

Temperature is set at 70 degrees F. Temperature is another variable.

Humidity is set at 60%. Humidity is another variable.

Scope height is set at 1.5 inches above the bore. This generally assumes a bolt-action rifle with a 40mm objective in medium rings. This changes the relationship between the bore line and sight line. While 1.5 inches is typical for a scoped rifle, iron sight are closer to the bore, and optically-sighted AR15s, AKs, FALs and any rifle with a large objective scope, are higher. These changes from muzzle to (far) zero are graphed further down.

Zero is set for three different yardages. Trace 1 is 100 yards. Trace 2 is 250 yards. Trace 3 is 500 yards.

ZEROED AT 3 DIFFERENT RANGES GRAPH - .308 Winchester Federal Gold Match 168-gr Boat-tail Hollow-point Match load
Image

You can see that using a 100 yard zero makes you hold over longer range targets a lot more once the bullet crosses past 230 yards or so. The bullet never rises any significant distance above the line-of-sight but drops alarmingly past 230 yards or so.

The popular 250 yard zero is a good compromise for most common hunting, or even combat, distances with the .308 cartridge where any type of longer range is anticipated. Rise above line-of-sight is not enough that you would miss a deer, being only about 4-inches or so at 150 yards, and the bullet gains another 90 yards before it drops 7-inches below the line-of-sight. Zeroing at 100 yards would require the bullet hit 3.5 inches above the point-of-aim (POA) to achieve a 250 yard zero.

The 500 yard zero is an interesting illustration. This would put you dead-on for shooting a known-distance target in competition at 500 yards. The obvious drawback to zeroing at such long ranges is for general use, whether combat or hunting is the excessive height of the bullet above the line-on-sight at normal ranges. At 250 yards it is 23 inches high! To attain such a zero at a normal 100 yard range, you would have to zero 13-inches higher than the bulleye/POA.

Now let's look at the same graph - only extended out to 1000 yards. You will be able to see the reason that the .308 Winchester with the common 168-grain Sierra BTHP Match bullet is considered to be an 800 yard sniper round. Even using the 500 yard zero, the bullet drops about 120 inches, or nearly 10 feet at 800 yards! Because this graph only goes to a drop of -160 inches (13.33 feet) you can't even see where it crosses 1000 yards.

1000 YARD GRAPH - .308 Winchester Federal Gold Match 168-gr Boat-tail Hollow-point Match load
Image

The general illustration information above is based on a scope height of 1.5 inches. Your iron sights would be lower and your scope height may be different by more than 1/2 inch - which causes a variation. If that is the case, the information would be fairly close, but more particulars like exact sight height and perhaps altitude would narrow in down a bit finer.

Checking the FAL in my safe, the iron sight height runs about 2.2". Further "guestimation" using a scoped AR15 to approximate a scoped AR10 gave a sight height of 3.2" Running that sight height against the 1.5" (bolt gun 40mm objective medium rings) height used in the graphs gives this difference up close trajectories and variations in the POI at 100 yards to achieve a 250 yard zero with Fed 168-gr Match:
Trace 1 (RED) 1.5 inches
Trace 2 (GREEN) 2.2 inches
Trace 3 (BLUE) 3.2 inches

3 Different Sight Heights Graph - Fed 168-gr GM / [color=red]T1 +1.5" / T2 +2.2" / T3 +3.2"[/color]
Image

Let's take a different view of the target and add a twist - the wind plus the effects of zero and path/drop.
Illustrated is a head-on view of a target AT 300 yards shot with the above load using the 250 yard zero. You will see the arc of the bullet path toward the target - striking 4.7 inches low - and you will also see the bullet drift from the effect of a 10-mph wind blowing 90 degrees from the right - that blows the bullet 6.9 inches to the left of aim. Point of Impact (POI) of the bullet on target is where the "scope crosshair" is at the 8 o'clock position. The increment numbers on the target rings are a bit confusing so ignore them.

[img width=640 height=540]http://i141.photobucket.com/albums/r64/friscopete/Ballistic%20Charts/308Targetview250ydzero.jpg[/img]

Notice how much a 10 mph wind will affect the bullet! While you can fairly easily adjust for elevation, judging the wind is another matter entirely and takes practice!
A wind blowing:
At five to eight miles per hour - leaves will rustle in trees and bushes.
At eight to ten miles per hour - small trees and large plants will bend. This is the amount of wind illustrated above.
Twelve to fifteen miles per hour - is enough wind force to sway large tree limbs and violently disturb small trees and bushes.
If a flagstaff is visible, the angle between the bottom edge of the flag and the pole divided by four will approximate the wind’s velocity at that point.

Next let's look at the effect of using different weight bullets in the same .308 on trajectory. We will stick with popular military or target styles again using:

Federal/American Eagle 150-grain FMJ-BT BC 456 - here we have a military 7.62 NATO M80 duplication load with a BC similar to the 168-gr Match bullet, but in a lighter bullet at a higher velocity - 2750 fps.
Federal Gold Match 168-grain BTHP (Sierra MatchKing bullet) BC 457 - the one we have been looking at before in all the graphs.
Federal Gold Match 175-grain BTHP (Sierra MatchKing bullet) BC 483 - this is the newest heavy-bullet sniper .308 load that shoots to the same velocity as the 168-gr Gold Match. You will be able to see the effect of a bullet with a higher BC at the same velocity.

250 YARD ZERO / 3 DIFFERENT BULLETS - Fed/AE 150-gr FMJ / Fed 168-gr GM / Fed 175-gr GM
Image

The amazing thing here is that all three loads have virtually identical trajectories when zeroed at 250 yards with the flattest trajectory going to the 150-gr FMJ, which, despite it's lower BC, started out faster and never relinquished the slight trajectory edge. In fact it is about 4 inches flatter shooting at 450 yards. If you were to extend the range to 1000 yards with this zero, drop would become 10 times what it is at 450 yards, or about 328 inches (27.33 feet)

Okay, now we will try the 500 yard zero for all loads with the range extended to 1000 yards.

500 YARD ZERO / 3 DIFFERENT BULLETS - Fed/AE 150-gr FMJ / Fed 168-gr GM / Fed 175-gr GM
Image

Here we have the "little" 150-gr FMJ still flatter shooting with a drop of around 105 inches (about 8.8 feet) at 1000 yards, while the next best is the 175-gr Match bullet with it's higher BC. However all are relatively close. Either way, range estimation becomes critical past 600 yards despite a 500 yard zero due to the steep drop of the projectiles, as the following shows:
150-gr FMJ = 800 yard drop - 105" / 1000 yard drop - 249"
168-gr HPBT = 800 yard drop - 120" / 1000 yard drop - 283"
175-gr HPBT = 800 yard drop - 115" / 1000 yard drop - 270"

1000 YARD WIND DRIFT / 3 DIFFERENT BULLETS - Fed/AE 150-gr FMJ / Fed 168-gr GM / Fed 175-gr GM
Image

This next graph shows the wind drift of all three bullets up to 1000 yards from the effects of a 10-mph sidewind from 90 degrees. Here we notice a bit different phenomena. The high ballistic coefficient of the 175-grain match bullet gives improves "wind-bucking" over the 168 match bullet, although it is basically the same as the 150 FMJ-BT M80-type bullet.
At 1000 yards we have the following amount of wind drift for each bullet:
150-gr FMJ = 92"
168-gr HPBT = 100"
175-gr HPBT = 92"
So where does the advantage of using the match bullets like the 175-gr lie, ballistically-speaking? In their accuracy superiority over standard FMJ ball ammo. Especially using the newer 175-grain Sierra MatchKing load, shooters have an extremely accurate bullet that will mimic the trajectory and wind drift of the cheap 150-gr ball ammo that sights are iron sights regulated for and is inexpensive to practice with.

What about sighting in with different bullet weights? You have to remember that each load acts individually on barrel vibrations or harmonics. Therefore one cannot just blindly assume that even if the loads have a similar trajectory, that they will print to the same POI on paper. The powder, powder charge weight, bullet weight and length can change vibration patterns. So always check each different load for POI and zero first. Often small adjustments may be necessary. Write these down so you can have the data to dial in your scope when changing to another load.

Next lets have some fun and look at three popular military calibers all zeroed at 200 yards and see how they stack up for bullet path. We will use the:
.223 Rem. 55-gr FMJ-BT 5.56 M193 duplication load by Winchester. BC .272
7.62x39mm 122-gr FMJ M43 duplication load by PMC. BC .292
.308 150-gr FMJ-BT 7.62x51 NATO M80 duplication load by Fed/AE. BC .456

3 CALIBER PATH COMPARISON - .223/55, 7.62x39/122, .308/150 all zeroed at 200 yards
Image

The .223/5.56 and .308/7.62 NATO loads are neck and neck in trajectory and much flatter than the slower a 7.62x39mm load. Even though the .223/5.56 has the lowest BC by far, the initial velocity of 3240 fps, which is much faster than either of it's two rivals, keeps it flat shooting. The .308 has a nice combination of decent velocity and a very high BC that makes it so ballistically balanced.

3 CALIBER ENERGY COMPARISON - .223/55, 7.62x39/122, .308/150 in foot/pounds
Image

Okay, this ought to make .308 shooter feel a lot better, because it shows why the .223 and 7.62x39mm are considered "medium-power" cartridges and the .308/7.62x51 NATO is a "full-power" cartridge. It also shows why the military consider both medium-power cartridges 300 meter rounds - when velocity, energy, and drop are considered.

Okay, enough ballistic comparisons. You could do this all day and dial-ups could never get it loaded! I hope you understand trajectory, sight-in or zero range, the effect of the wind on bullets, and all the rest of the exterior ballistic stuff and know more about how to apply it for your rifles and cartridges.

ONLINE or DOWNLOADABLE BALLISTIC PROGRAMS
Handloads.com: www.handloads.com/calc
Nikonhunting.com: www.nikonhunting.com/SpotOnRegistration
HuntingNut.com: huntingnut.com/index.php?name=PointBlank

If you know of others, please PM me and I will add them to the list.

Post reprinted with explicit consent of Frisco Pete, et al. Copyright 2014, http://www.sksboards.com Picture provided by http://www.fubarhill.com.
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