Originally Posted by
MistWolf
While the gases do push against the bolt, they cannot push the bolt forward! Although the bullet has exited the barrel by the time the expansion chamber fills with enough pressure to start the carrier moving, the pressure in the bore is still greater. Also, the spent case has expanded enough to take up what little slack there was and resist any forward motion of the bolt.
It's also a fallacy to believe the AR does not have a piston. It's piston is part of the bolt and the carrier is a cylinder. The piston is inline with the bolt and bore, reducing muzzle climb induced by off center recoil forces and reciprocating mass.
The AR does not feature primary extraction, a mechanical camming action that starts extraction as the bolt begins unlocking. Many other designs do, such as the M14 and most bolt actions. Whether the AR uses the original inline piston or an aftermarket offset piston relocated to the gas block, it must rely on "brute force" to yank the spent cartridge from the chamber