Abstract: A magazine-fed firearm has a frame, a barrel connected to the frame and chambered for a cartridge of a first selected specific type, the frame having a magazine well configured to receive a first magazine type configured to contain cartridges of the first selected specific type, the frame including a compatibility facility operable to facilitate operational presence in the magazine well of the first magazine type, and the compatibility facility operable to deny operational presence in the magazine well of a second magazine type configured to contain cartridges of a second specific type different from the first specific type and incompatible with the barrel. The first and second magazine types have a common dimensional characteristic compatible with the magazine well except for interface with the compatibility facility, such that without the compatibility facility, the first and second magazine types are otherwise compatible with the magazine well.

Abstract: Provided is a firearm lower receiver configured to mate with an AR-pattern, auto-loading upper receiver assembly. The lower receiver has a body and a hinge mechanism having a fixed part integral to the lower receiver body and a moving part to which a buffer tube is mounted. The hinge is movable between an extended, operating position where the buffer tube is substantially aligned with the bolt carrier and a folded position. The fixed part includes a carrier blocking member movable between a blocking position where the blocking member prevents rearward movement of a bolt carrier in the upper receiver and a displaced position. The blocking member is spring biased toward the blocking position. A striker extends from the moving part and is configured to contact the carrier blocking member and move it against the spring bias to the displaced position when the hinge mechanism is in the extended, operating position.