18 U.S.C., § 921(A)(7) and 27 CFR § 478.11 The term "Rifle" means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of the explosive in a fixed metallic cartridge to fire only a single projectile through a rifled bore for each single pull of the trigger.
Muzzle loaders are not even considered firearms so the rules do not apply. That is why you can buy and send them through the mail and they do not have to be registered.