A main chamber feeds a firing chamber, with the latter being completely emptied for every shot. While it certainly works, this system is very wasteful. Not only is air still being fed through the barrel long after the projectile has left the muzzle, but the average pressure in the barrel is bound to be low unless an impractically large firing chamber is used.
The ideal valve therefore would open and close very quickly, before the projectile has left the muzzle. A hammer valve is a practical way of achieving it, which is why it is the most commonly used valve in commercial pneumatic airguns. They are difficult to scale up however, so the compromise for "spudgun" sized calibers would be a hammer valve used to pilot a quick exhaust valve on this principle:
I plugged some numbers into GGDT to make the point, for a 3/4" caliber launcher firing 30 gram projectiles at 150 psi. One has a chamber with the same volume as the barrel that it dumps for every shot, while another has a chamber 10 times the volume of the barrel, with a valve that closes when the projectile is about 50% of the way down the barrel.
Even though the valve is open for a shorter interval, the average barrel pressure is higher in the case of the timed valve with the larger chamber, resulting in a higher velocity. The barrel pressure at projectile exit is also lower, meaning the launcher will also be quieter, a consequence of its greater efficiency.
In terms of gas usage, let's say we started with a 106 cubic inch chamber at 150 psi in both cases, with this chamber feeding the 10.6 cubic inch firing chamber in the case of the dump valve launcher.
To fill the firing chamber in the first case, the pressure would have dropped to 136 psi in the main chamber, as s indeed it would have in the firing chamber, so actual energy would have been even less, 64.2 ft lbs as opposed to 70.8 ft lbs at 150 psi. For a more direct energy comparison however, I used the same firing pressure for the GGDT simulation.
In the second case, the pressure drop is only 8 psi to 142 psi. This means that in the case of the timed valve, we only used about half the air that we used for the dump valve, and yet obtained a performance increase of around 20% more muzzle energy.
Here is a theoretical 5 shot comparison, giving the dump valve the edge with the firing chamber already being filled to 150 psi, with firing pressure and resulting projectile muzzle energy:
Dump Valve
Shot 1) 150 psi 70.8 ft lbs
Shot 2) 136 psi 64.2 ft lbs
Shot 3) 124 psi 58.5 ft lbs
Shot 4) 113 psi 53.3 ft lbs
Shot 5) 103 psi 48.4 ft lbs
Timed Valve
Shot 1) 150 psi 77.2 ft lbs
Shot 2) 142 psi 71.3 ft lbs
Shot 3) 135 psi 66.1 ft lbs
Shot 4) 128 psi 61.0 ft lbs
Shot 5) 122 psi 56.6 ft lbs
The difference is quite dramatic and something to consider when designing pneumatic repeaters.
