The portion of a muzzleloader rifle that fits against your shoulder is the

The portion of a muzzle-loader rifle that fits against your shoulder is the - BUTT.

A muzzle-loader is a firearm in which the projectile and usually the propellant charge is loaded from the muzzle of the gun. This is distinct from the more popular modern designs of breech-loading firearms. That portion of a muzzle-loader rifle that fits against ones shoulder is called the butt.