Easy semiauto airgun

[sharpshooter]

My air chamber is an 8" length of 1" galvanized pipe with an ID of 1.049 inches, so yes it is a fairly large chamber for high volume.

[Forum Admin]

That's a very large chamber for the size of barrel. With the right flow and speed on the valve combined with a long enough barrel, 500+ fps should be doable on 165 psi. Judging from the penetration at 50 feet, it sounds like you are shooting in that ballpark.

On this link: http://www.doomlabs.com/projects/caselman/caselman_flup/The%20Caselman%20air%20powered%20machine%20gun%A0%B7%A0Flup%203.html, you can find a way to mathematically estimate your speed. The author makes a bad assumption and overestimates his valve efficiency. In real world experience, the most valves I have observed hit between 25 and 45%. You really have to sweat to get close to 50% efficiency. While this wont give 100% accurate muzzle velocities, it will be a good estimate.

[sharpshooter]

The large air chamber was first for built for the .625 barrel with darts and then for the choked shotgun barrel and I simply used the same air chamber and stock setup to test the next two barrels I built, the .311 and .200. I do like the results I`m getting with both the large and small bore barrels. I believe the large volume helps in the speeds I think I`m getting as well as the penetration I know I`m getting.

[Forum Admin]

The large chamber does wonders in increasing power as it reduces the pressure drop in the barrel. I once built a PVC airgun with a overkill air chamber, it was capable of propelling a .58 roundball to 100 foot pounds of energy. That was grossly overpressurized to 175 psi in my younger foolish days 

[mouz]

well, that is a huge chamber.... mine is 5 inch long 1/2" iron pipe lol, also i use ggdt for this, which has never been said to be perfect. i do get around 1/2" penetration into white pine boards though with the steel. and yes, big chambers really are amazing, as has been proved by my copper .56 cal(which can and will go super sonic with a .5 gram dart, though that projectile has no energy at all and a terrible ballistic coefficient) also you might have a longer barrel than me, which makes a big difference.