Gas System
The mechanism in a semi-automatic firearm that taps propellant gas to cycle the action — common AR-15 lengths are carbine, mid, and rifle.
Most modern semi-autos are gas-operated: as the bullet passes a port in the barrel, some high-pressure gas is tapped off to drive the bolt back, ejecting the spent case and chambering a fresh round. Designs vary — direct impingement routes gas back through a tube (the classic AR-15), while a piston system uses gas to shove an operating rod.
On the AR-15, the distance from chamber to gas port defines the system length — carbine, mid-length, or rifle. A longer system generally gives a smoother, softer cycle and runs cooler, while shorter systems suit shorter barrels. Proper gas tuning balances reliable cycling against excess recoil and wear.