Abstract: A muzzle loading firearm has a barrel bore into one end of which is force fitted a breech plug. The barrel wall has a radial aperture through one side and a diamaterally opposed recess in its inner surface, and the breech plug has a diamaterally extending passage aligned with the aperture and recess in the barrel wall. In one embodiment, a bolster is force fit into the barrel aperture, breech plug passage and barrel recess. The breech plug has a bore in its inner end communicating with the passage therein, and the bolster has a flash chamber and passage aligned with the breech plug bore.