The MAG's receiver is constructed from sheet metal stampings reinforced by steel plates and rivets. The front is reinforced to accept the barrel nut and gas cylinder which are permanently mounted. Guide rails that support the bolt assembly and piston extension during their reciprocating movement are riveted to the side plates. The bolt's guide rails are shaped downward to drive the locking lever into engagement with the locking shoulder, which is also riveted to the side plates. The rear of the receiver has been reinforced and slotted to accept the butt-stock.