A muzzle-loading firearm includes a breech plug for mating, non-threaded engagement with a housing located at the breech end of an axial bore of the firearm barrel. The plug includes a cylindrically-shaped body member having opposite first and second ends and an outer surface between the ends and configured so that when the plug is positioned within the housing, the outer surface is spaced close to the inside surface of the housing to facilitate insertion and removal of the plug from the housing. The body member is formed with a primer chamber for receiving and retaining a primer or a percussion cap at the first end, and a passageway for fluidly communicating the primer chamber with a powder charge at the second end. The body member may have a powder chamber for receiving and retaining at least part of a powder charge at the second end when the firearm is loaded. The material and dimensions of the body member may be such that the body member deforms radially outwardly so that the outer surface of the body member and the inside surface of the housing form an essentially gas-tight seal during firing. The powder chamber contains much of the powder residue generated by the firing. In order to insure that the muzzle-loader cannot be readily used as, or converted to, a breech-loading firearm, different muzzle-loader/breech plug design features are presented. With one design feature, the maximum cross-sectional inner dimension of the housing is less than the diameter of the bore of the barrel. With another design feature, alignment of the firing pin and primer chamber (or hammer and percussion cap holder) is along an axis not centered with the axial bore of the barrel. The cross-sectional shape of the housing and breech plug may be non-cylindrical. For example, the cross-sectional shape can be oblong, essentially triangular, etc. Thus, conventional, breech-loading ammunition will not be effectively usable in the muzzle-loading firearm.