A breech plug for a muzzle loading firearm is attachable to a barrel having a muzzle end and a breech end. A breech element having a breech face is connected to the breech end of the barrel. The breech element moves between an open position in which the breech face is away from the breech end of the barrel, and a closed position in which the breech face abuts the breech end of the barrel. A breech plug body is removably attachable to the breech end of the barrel. The breech plug body has a rear face that abuts the breech face when the breech element is closed, so that a primer contained in a primer pocket in the rear face of the breech plug body is fully supported on all sides.