Abstract: Firearm of muzzle-loading type is constructed to provide ready attachment of different barrels, such as a rifle or shotgun barrel, having different sizes to the same butt stock. The butt stock is provided with a mounting plate on its front end which is threadably engaged by an adapter member secured to the breech end of the barrel. The adapter member has a rear plate portion which matches the shape of and is co-extensive with the mounting plate, and abuts the same in operative position. Yieldable locking means on the adapter member provides for ready locking and unlocking of the adapter member and mounting plate. Assembly of the barrel to the butt stock is made by screwing the adapter member into the mounting plate and locking the parts together in operative position.