View Full Version : Large Caiber Airguns - tech question

04-02-2008, 06:42 PM
To those of you running .30+ caliber air rifles; At what "chamber pressure"(the pressure behind the bullet upon firing) do they operate, and what is the volume of air released upon firing? I'm considering making a .355-7 cal airgun on a .357 barrel blank I've got laying around and would appreciate data on these figures so I can design the valve accordingly. Planning on using HPA, but CO2 is a possibility as well.

I remember hearing somewhere that tank pressure in a CO2 tank is right around 800 and HPA can be all the way up to 4500, but I'm not sure about operating pressures. I'm not worried about the barrel being able to take the pressure, but I want to make sure the action and bolt locking areas are up to it.

Suggestions on bullet weight would be helpful as well. I'd like to push 1000 fps w/ a 60-90 ish grain bullet(135gr pure lead RN, drilled to be hollow base @ appropriate weight). Barrel length is 20", chambered in .357 Mag w/ 1 in 19 or so. Is this velocity even possible? Google only provides so much info.


04-02-2008, 07:33 PM
I'll answer the last question first. No. My largest caliber is 9MM and it operates on a rather standard 3000psi. With a 70 gr pellet I can get about 800 fps and that is with a longer barrel. Your short barrel and heavy projectile both work against you. I've shot up to 20MM airguns and although they make lots of power, the velocity is still not there. Start small on your valve, you'll be suprised at much flow you will get. Increase your valve size slowly and watch your velocity curve to make sure your hammer weight and spring are operating the valve efficiently. Lighter on the hammer is better as it lowers the operating presure.

04-03-2008, 09:33 AM
I have never built my own air rifle but have been toying with the idea. Don put me on to a really knowledgable group of guys out in California that have tried just about everything imaginable.

I have learned so much from their post that I can almost have an intelligent consversation about air gun performance and at least have learned enough to recoginze that there are some unbeliable techno-wizards out there building some amazing rifles.

Take a look at some of the post at

There's years worth of learning to be picked up in a few days worth of reading.

While your building that souped up large caliber rifle, gather up your air tank and a pocket full of small pellets and come on to a match at shoot with us at 25 yards.

04-04-2008, 06:25 PM
Thanks guys, it's a start. PM's may follow.