Abstract: A firearm comprises a barrel having a bore and a receiver forming an ignition chamber. A bolt is slidable in the ignition chamber and includes a striker projection for igniting a primer of the projectile. A bolt advancing mechanism comprises an arm having a first end mounted on the bolt such that a second end of the arm is movable laterally relative to the fore-aft axis. A boss is carried at the second end of the arm. An actuator member is movable in the fore-aft direction relative to the bolt and boss, and includes a first cam surface inclined relative to the fore-aft axis for urging the boss forwardly and laterally. The boss is movable laterally along the first cam surface in response to the boss becoming aligned with a lateral groove in the receiver. Such lateral movement of the boss is in a direction tending to displace the bolt forwardly.