A breechblock assembly includes a breechblock configured to reciprocate along a top of the firearm receiver between a recoil position and a battery position. An actuator is coupled to the breechblock and is configured to move between first and second positions. A lever on the breechblock in contact with the actuator is movable between a blocking position and a non-blocking position. For example, the lever is generally oriented along the central axis and extends rearward beyond a proximal face of the breechblock. When the actuator is in the first position, the lever is in the blocking position. When the actuator is in the second position the lever is in the non-blocking position. When assembled in a firearm, an end of the lever is positioned to block the hammer from striking the firing pin when the breechblock is out of battery by more than a predefined distance.