Retaining devices securable on firearms are selectively operable to engage and retain a bolt assembly in a substantially-fixed position for an indeterminate period of time. The retaining devices include a mounting plate securable to the receiver and a retention pin that extends through a passage in the mounting plate. The retention pin extends axially between first and second ends that project outwardly beyond the mounting plate. The retention pin is supported on the mounting plate for axial displacement between a first pin position and a second pin position. In the first pin position, the second end of the retention pin engages and retains the bolt assembly in a substantially-fixed position relative to the receiver. In the second pin position, the second end of the retention pin is disengaged from the bolt assembly.