When fired, propellant gases from the ignited cartridge following the projectile down the bore are vented through a gas port into a gas cylinder where they drive the piston and bolt carrier rearward. There is about of uninterrupted free travel calculated to permit the gas build-up in the bore to drop to a safe level before the carrier's cam slot rotates the bolt counter-clockwise to unlock. The locking lugs have no pitch therefore no primary extraction occurs during the unlocking sequence. A deeply seated spring-loaded claw extractor in the bolt head extracts the spent cartridge casing from the chamber and a spring-powered ejector fixed to the front feed mechanism trigger housing ejects the casing. The bolt carrier continues to the rear and compresses the captive recoil spring on its guide rod.