A muzzleloader firearm has a barrel with at least one lug retaining structure that includes an internal annular groove and an internal annular shoulder that is both adjacent the annular groove and positioned between the annular groove and the breech end of the barrel. Each annular shoulder is provided with lug entry cutouts, which are radially spaced about the shoulder. The firearm also has a breech plug that is provided with multiple lugs forming at least one discontinuous external shoulder. The lugs are spaced and sized so that they align with the lug entry cutouts. Preferably, the number of external shoulders on the breech plug match the number of internal annular grooves in the barrel. The breech plug is installed in the barrel by aligning the lugs with the lug entry cutouts, and then rotating the plug so that the lugs are no longer aligned with the lug entry cutouts.