Abstract: A breech plug for a muzzle-loading firearm is disclosed wherein the breech plug includes a soft metal insert to provide a soft metal-to-steel seal between the breech plug insert and the barrel of the firearm to prevent blow-by. The insert also provides for a soft metal-to-steel engagement between the forward end of the nipple and the breech plug insert to prevent blow-by when the powder charge on the nipple is ignited.