A firearm receiver system including a receiver body, an ammunition belt feeding assembly and a bolt carrier. The receiver body includes a first face and a second face approximately opposite the first face. The receiver body is configured for having means for triggering engaged therewith at the first face and for receiving belt-fed ammunition through the second face. The ammunition belt feeding assembly is mounted at least partially on the receiver body and the ammunition belt feeding assembly is configured for supplying the belt-fed ammunition to the receiver body through the second face. The bolt carrier is movably mounted on the receiver body between the first and second faces of the receiver body and the bolt carrier is configured for being actuated by gas-energized piston-driven means.