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Hi, has anyone here got any experience in building air rifles? Specifically on the vale metering and design. My goal is .70 cal and at least 800 ftlbs of energy.
I have a dive compressor to fill to 4000 psi and Paintball gun air tanks are cheap.. I dont care if i only get one shot per tank..
The theoretical maximum FPE can be found from E = F x D, where F, the force in lbs., is the bore area (sq.in.) times the pressure (psi) and D, the distance that force is applied through, is the barrel length in feet.... A .70 cal has a bore area of (0.70 x 0.70 x PI/4) = 0.385 sq.in, which at 4000 psi would produce a force of (0.385 x 4000) = 1540 lbs.... If the barrel was 2 feet, at 100% efficiency, with an infinite sized reservoir and no losses, that works out to (1540 x 2) = 3080 FPE.... Of course in the real world we don't even come close to that, the best of PCPs have great difficulty reaching 50% of that value, and 40% is more common, even for high performance PCPs.... 40% of that is 1232 FPE, with a 24" barrel and a very large reservoir capable of maintaining close to that 4000 psi throughout the shot....
A .70 cal barrel that is 24" long has a bore volume of (0.385 x 24) = 9.24 CI = 151 cc.... If your air reservoir was the same volume, and the valve stayed open until the bullet left the muzzle, the pressure at that point would have dropped by 50%, from 4000 psi to 2000 psi, and reduced the average pressure to 3000 psi, with the average force dropping proportionately.... That could reduce the FPE from 1232 FPE to 900-1000 FPE.... I am giving you these figures just to make you think of some of the problems that are involved in building a large caliber PCP to produce high FPE....
To your question, maximum power is obtained when you have caliber sized passages throughout the system.... Obviously you can't drill a 0.70" hole in the bottom of a .70 cal barrel or the bullet will tend to drop into it and may be damaged on loading.... To get a full area barrel port you can either use multiple ports, or an oblong port.... The largest you can go with a simple drilled port is 75-80% of the caliber.... With careful deburring, you should be able to use a 9/16" (0.563") barrel port, with matching transfer port and valve exhaust port.... Most conventional valves use a stem on the poppet, which partially obscures the valve throat, so you have to make the throat larger to allow for that.... If we assume the stem on your poppet is 3/16" (0.188"), you should use a valve throat of about 5/8" (0.625")…. The head of the poppet would then end up at about 11/16" (0.688") to have a wide enough seat margin to survive the forces involved, and I would use PEEK for the material, Delrin will not survive at that diameter and pressure....
In case you haven't thought about the forces involved in opening such a valve, with a seat diameter of 0.688", at 4000 psi, it takes (0.688 x 0.688 x PI/4 x 4000) = 1486 lbs. of hammer strike to crack the valve off the seat.... In addition, the hammer must have enough energy and momentum to provide the lift and dwell required to dump 150 cc of air through the valve in less than 3 mSec.… You may want to consider using a spool valve which requires only a fraction of the force to crack open, and once cracked will "blow open" and dump the entire reservoir.... While this is wasteful of air, you stated you don't care if you only get one shot per fill, and the greatly reduced hammer strike required may be necessary....
Bob