>a piston gun is generally much better than impingement. The AR-15 is a piston gun, though: the piston is the bolt itself (the bolt carrier acts as its cylinde

>a piston gun is generally much better than impingement. The AR-15 is a piston gun, though: the piston is the bolt itself (the bolt carrier acts as its cylinder walls- this is the reason why the back of the bolt has piston rings). And because said piston is right in the center of the moving parts, you don't have to worry about those parts rotating off their axis (i.e. there's no force tilting them up or down)- the only reason the receiver needs to be there is to hold the barrel on. This doesn't work for barrel lengths below about 10.5"; at that point an external piston system is indeed more reliable. Also, because the gas tube is a weak point in terms of heat dissipation, the internal piston AR isn't the best choice for sustained automatic fire- but in that case you just give one squad member an external piston gun much like how the Russians distributed their beefed-up AKs.