For use on a firearm having a removable ammunition magazine, a longitudinally reciprocating bolt, and a pivoting bolt catch member, the present invention provides an empty ammunition magazine bolt hold open mechanism having a longitudinally extending axel pivotally mounted to the firearm for rotation about a longitudinal axis. The axel has a forward lever arm that extends transversely from the axis and is positioned to be engaged by a magazine follower when the magazine is empty of ammunition. Displacement of the forward lever arm causes rotation of the axel. The axel has a rear portion configured to engage the bolt catch and cause pivotal movement of the bolt catch member when the axel is rotated.