Shortening an air rifle barrel.

For all pcp's and springers. Tuning, repairs, pellets, show and tell.
Cedric
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Joined: 18 Apr 2014, 00:08
Location: Lancashire, UK

Shortening an air rifle barrel.

Post by Cedric » 13 May 2014, 18:28

Just bought a HW99S and it's a bit long, no mod. fitted yet but if I do it will be like a drainpipe !

I believe Sandwell Field Sports are shortening these barrels to 10 1/2" plus a tune and marketing them as the "Imp" model so I presume the shorter barrel does not result in lack of performance.

Does anyone have experience of shortening barrels and what the implications might be?

biffo
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Joined: 01 Mar 2014, 13:05
Location: Surrey

Re: Shortening an air rifle barrel.

Post by biffo » 13 May 2014, 20:51

Read this:

http://www.kiledjian.elac.org/phys%2000 ... %20Gun.pdf

Bear in mind that noise levels (spring, piston, muzzle blast etc) will also increase.

Bif

Cedric
Posts: 11
Joined: 18 Apr 2014, 00:08
Location: Lancashire, UK

Re: Shortening an air rifle barrel.

Post by Cedric » 15 May 2014, 21:36

Interesting paper, I noted he discounted calculating spring piston rifles as they were too complicated ! :problem:
His conclusions - Longer barrels mean more energy diverted to friction, whereas shorter barrels mean more gas pressure is wasted.
Once the pellet leaves the barrel, the remaining gas pressure is dissipated in the air. So, the optimum barrel length is a trade-off between friction and lost gas pressure.


It would be interesting to find out if these forces cancel each other out.

Gren
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Location: Thames Valley

Re: Shortening an air rifle barrel.

Post by Gren » 16 May 2014, 03:10

I got involved with all this madness a few years ago, and you will need a degree in Physics and the ability to write programs (or be a wiz with spreadsheets) to approach it from the mathematical angle. I have all the qualifications, but there are soooo many variables to consider. Experimentation through trial/error is probably your best bet, and plagiarizing the experts is a good place to start ;-)

Sandwell Field Sports, well, they know more than the average punter, but not much IMHO. John Bowkett has a good rep for his work, as does Steve Pope (aka Venom); I'd Google them and see what they have done.

If you are a member of AirgunBBS you'll find a bit of info on the site, but most people don't really have a clue! BuddyBoy did some great work on the AA TX200 a few years back, that would be worth reading if it's still on their database.

Your biggest gains through shortening the barrel will probably be in reduced lock-time and changing the barrel harmonics. These can both have a huge effect on accuracy. If you don't shorten it by more than say 30% I don't think you will notice any loss in power as that is the high speed (high friction) vs low pressure part of the cycle. If you have access to a lathe then you could have "fun" by taking 10mm off at a time and testing accuracy & power. Best done with a tatty 2nd hand barrel until you find the sweet spot. Crowning is very important after each chop, and that is a subject in itself !

I've wondered off your original question.. yes cut it down to a comfortable length (see TX200 carbine) and you will be fine (probably).

Good luck, I'd be interested to know how you progress.

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rodp
A bad influence
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Joined: 09 Mar 2012, 22:49
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Re: Shortening an air rifle barrel.

Post by rodp » 16 May 2014, 21:00

Gren wrote:I got involved with all this madness a few years ago, and you will need a degree in Physics and the ability to write programs (or be a wiz with spreadsheets) to approach it from the mathematical angle. I have all the qualifications, but there are soooo many variables to consider. Experimentation through trial/error is probably your best bet, and plagiarizing the experts is a good place to start ;-)

Sandwell Field Sports, well, they know more than the average punter, but not much IMHO. John Bowkett has a good rep for his work, as does Steve Pope (aka Venom); I'd Google them and see what they have done.

If you are a member of AirgunBBS you'll find a bit of info on the site, but most people don't really have a clue! BuddyBoy did some great work on the AA TX200 a few years back, that would be worth reading if it's still on their database.

Your biggest gains through shortening the barrel will probably be in reduced lock-time and changing the barrel harmonics. These can both have a huge effect on accuracy. If you don't shorten it by more than say 30% I don't think you will notice any loss in power as that is the high speed (high friction) vs low pressure part of the cycle. If you have access to a lathe then you could have "fun" by taking 10mm off at a time and testing accuracy & power. Best done with a tatty 2nd hand barrel until you find the sweet spot. Crowning is very important after each chop, and that is a subject in itself !

I've wondered off your original question.. yes cut it down to a comfortable length (see TX200 carbine) and you will be fine (probably).

Good luck, I'd be interested to know how you progress.

I've got a chop saw and 3 "A" levels in Black Country engineering (that ay level, that ay level and that one ay level) Any use for the job ?

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Davie_M
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Joined: 16 Jun 2013, 05:54
Location: Hamilton

Re: Shortening an air rifle barrel.

Post by Davie_M » 16 May 2014, 21:13

might be wrong but i am sure air gun barrels are choked at the muzzel

sonic
Posts: 34
Joined: 05 Apr 2013, 08:03
Location: Earby Yorkshire/Lancashire border

Re: Shortening an air rifle barrel.

Post by sonic » 18 May 2014, 17:33

Davie_M wrote:might be wrong but i am sure air gun barrels are choked at the muzzel
Your not wrong they are choked.

Cedric
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Joined: 18 Apr 2014, 00:08
Location: Lancashire, UK

Re: Shortening an air rifle barrel.

Post by Cedric » 22 May 2014, 22:51

I've heard about some barrels being choked. I'd be fascinated to learn how the manufactures achieve that.

sonic
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Joined: 05 Apr 2013, 08:03
Location: Earby Yorkshire/Lancashire border

Re: Shortening an air rifle barrel.

Post by sonic » 23 May 2014, 00:19

Quite easily with a hammer forged barrel, the mandrel with rifling on it has the taper. Which is transferred to the barrel as it is hammered round it. Sometimes the barrel is squeezed after rifling. There is an air rifle with a smooth bore that's rifled for the last few inches by using a hydraulic press I believe that it is choked as well.

Cedric
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Joined: 18 Apr 2014, 00:08
Location: Lancashire, UK

Re: Shortening an air rifle barrel.

Post by Cedric » 28 May 2014, 00:30

:D All becomes clear ! So, hammer forged barrels are forged round a mandrel which has rifleing in it, not rifled later. Clever buggers !!

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