A muzzle loading firearm includes a barrel having a bore and a breech plug closing the breech end of the bore. An aperture in the barrel adjacent the breech end is adapted to mount a bolster which extends into the breech end of the bore. A flash transfer orifice communicates with a flash chamber in the bolster which in turn communicates with the breech end of the bore to permit firing of a powder charge contained therein. The firing chamber has an end wall located transversely opposite the aperture so that firing forces which would otherwise tend to bias the bolster outwardly of the aperture are resisted by firing forces acting on the end wall. Rearward biasing of the bolster against the breech plug by the firing forces is resisted by seating of the bolster within the aperture.