A muzzle loading firearm with a break-open action includes a rear stock assembly and a barrel assembly. The barrel assembly includes an elongate barrel, having a bore with muzzle and breech ends, and a breech plug removably connected the breech end of the barrel, which encloses a chamber at the rear of the bore. The barrel assembly is pivotally attached to the rear stock assembly, and is movable with respect to the stock assembly between an open position in which the breech end is accessible for service, and a closed position in which the breech end is inaccessible and secured for discharge. A lever arm, optionally configured to serve as the firearm trigger guard, is user actuated for moving the action between the open and closed positions. The breech plug is configured for rapid removal from the barrel by hand when the action is open.