[m102404]

The negative pressure gas system doesn't work on a fixed amount of gas per shot...it's entirely dependant on the weight of the BB and the length of the inner barrel.

(ref. there's a very good little write up on Redwolf's site somewhere about how NP and PP systems work...diagrams and all)

Basically...there's two halves to the firing cycle so far as gas is concerned. Part1 - Firing...and Part 2 - Recoiling.

Part 1 is where the knocker knocks the mags main valve and gas is released up into the nozzle (open bolt or not). The gas is forced forwards since there is less pressure ahead of the BB (the atmostphere) vs. behind it in the bolt (open or not). Since the knocker is only retracted when the bolt racks back, it will stay forward and gas will continue to flow behind the BB as it travels down the barrel.

As soon as the BB exits the barrel there is now more pressure behind the float valve in the nozzle (open bolt or not) vs. in front of it. So the float valve is pressed forward, plugging the nozzle and pressure builds behind it in the brass tube/nozzle body. That starts Part 2.

Part 2
The gas expanding in the sealed spaced builds pressures and eventually overcomes the recoil spring of the bolt...the bolt travels rearward...the hammer is cocked on the way back and somewhere during it's travel it trips the knocker release (part #66 in the M4). The knocker retracts....gas from the mag is cut off....the bolt continues to recoil by remaining pressure and momentum. Bolt carrier bottoms out....and is slung forward by the bolt spring.

So.......with a NP system it will fire a BB out no matter how long the inner barrel is. It could be 1cm....or 1m long...doesn't matter. Gas will flow in/down the barrel until the BB leaves the muzzle. Obviously this will use up more gas per shot and you'll typically get very high muzzle velocities.