 Here, the gas tube protrudes into a "gas key" (bolt carrier key), which accepts the gas and funnels it into the bolt carrier.At this point, the bolt is locked into the barrel extension by locking lugs, so the expanding gas forces the bolt carrier backward a short distance.As the bolt carrier moves toward the butt of the gun, the bolt cam pin, riding in a slot on the bolt carrier, forces the bolt to rotate and thus unlocks it from the barrel extension.