Abstract: A bolt hold open (BHOA) catch assembly configured for operating with rimfire ammunition for use with a firearm including a receiver with an exposed receiving chamber. A BHOA housing is supported upon the lower receiver in communication with a receiver chamber in communication with a bolt assembly. A catch is supported in elevatable fashion relative to an end of the actuator housing and an end most and downward extending structural portion of the elevatable catch is capable of being manipulated by an externally configured and toggled bolt catch so that an inner pivoting end portion thereof engages the structural portion to raise the BHOA supported catch into contact with a forwardly traveling bolt, such as upon release of a previously retracted charging handle. The BHOA catch can alternatively be actuated upwardly by a suitably configured follower associated with an attachable and cartridge holding magazine, this again following a last shot hold open protocol.