Abstract: Provided is a last round bolt hold-open mechanism for a firearm with a removable ammunition magazine and a longitudinally reciprocating bolt that automatically releases the bolt when a loaded ammunition magazine is inserted. The mechanism includes a frame mountable to a firearm receiver and having a pair of mounting flanges. A catch member is pivotally mounted on a pivot axle oriented transverse to the longitudinal reciprocation of the bolt and carried by the mounting flanges. The catch member has a rearward portion that moves between bolt blocking and bolt release positions and is spring biased toward the bolt blocking position. A forwardly extending finger is configured to be lifted by the presence of at least one ammunition cartridge in the magazine and, when lifted, causes the catch member to pivot to the bolt release position.

Abstract: A multi-column box magazine for rimmed ammunition cartridges of varied lengths has an elongated housing with a neck portion where cartridges are arranged in a single column and a mouth at an upper feed end through which cartridges are inserted and extracted. In a multi-column portion, cartridges are arranged in laterally alternating columns, and in a transition portion the alternating columns are transitioned into the single column as the cartridges are moved toward the mouth within the housing. A follower is biased by a spring toward the mouth. The multi-column portion has a forward interior surface limiting forward movement position of cartridges, which varies depending on individual cartridge length, and the neck portion having a forwardly sloped rearward wall which confronts the cartridge rim and shifts the cartridge forward, as needed, as cartridges are moved upwardly within the housing to the mouth.