What exactly does the gas piston on the AUG or USR do? The FAL piston drives the operaitng rod back to cycle the bolt. The setting allows gas to excape due to the setting. When too much gas is exhausted, the gun will not cycle. When too much gas is kept in the system, it gives the bolt a hard knock against the back of the receiver. That will cycle the bolt, but will give more wear to the back of the receiver. Does the gas piston on the AUG/USR limit the gas so it will cycle the bolt? There is no operating rod on the AUG/USR. It must limit the gas pressure down to a point to where the bolt will unlock, Right? On the AR15, It cycles the bolt. On the AK47, although it doesn't have a gas piston, the gas pressure drives the [operating rod] to cycle the bolt also. There is no adjustment on this. Let's all jump in here and revive this forum! Uzi Geo. Thank's.