Short-stroke piston

With a short-stroke or tappet system, the piston moves separate from the bolt group. It may directly push[6] the bolt group parts as in the M1 Carbine or operate through a connecting rod or assembly as in the Armalite AR-18. In either case, the energy is imparted in a short, violent push and the motion of the gas piston is then arrested allowing the bolt carrier assembly to continue through the operating cycle through kinetic energy. This has the advantage of reducing the total mass of recoiling parts. This, in turn, enables better control of the weapon due to less mass needing to be stopped at either end of the bolt carrier travel. These sudden stops on other systems disrupt the weapon's point of aim, especially with light weapons in full-automatic fire. The primary disadvantage of this system is that it relies more heavily on spring pressure and less on bolt mass for the kinetic energy to chamber a round and lock the breech.

long-stroke piston

With a long-stroke system, the piston is mechanically fixed to the bolt group and moves through the entire operating cycle. This system is used in weapons such as the Bren light machine gun, AK47, and M1 Garand. The primary advantage of the long-stroke system, beyond design simplicity and robustness, is that the mass of the piston rod adds to the momentum of the bolt carrier enabling more positive extraction, ejection, chambering, and locking. The primary disadvantage to this system is the disruption of the point of aim due to the center of balance changing during the action cycle and energetic and abrupt stops at the beginning and end of bolt carrier travel.