Abstract: A muzzle-loading firearm with a lever-operated breechblock pivotally movable between a securely locked closed position to an open position to expose in plain view a removable breech plug having a cavity for receiving a primer, detonation of which causes ignition of a powder charge in the muzzle loader barrel. The breechblock and a lever are pivotally mounted on a rearward barrel lug which is attached to the barrel. The lever is operatively connected to the breechblock by a pivotally mounted link. The pivotal link in an angular position exerts a locking force on the closed breechblock. The lever manually moves the breechblock rearwardly and downwardly to an open position. A firing mechanism assembly, including a trigger, hammer, firing pin, and torsional springs for forwardly biasing the trigger and hammer, is a unitary member of the rotating breechblock structure.