The term “antique firearm” means—
(A) any firearm ...
Why you cannot or will not understand this fact eludes me.
If you don't believe me, ask your lawyer. Or ask the BATF. Whether you care to admit it or not, muzzleloaders are NOT firearms, are not subject to the taxation and licencing restrictions of firearms, and are routinely sold in legal interstate commerce by outfits not having an FFL to private citizens not having an FFL.
These are facts. Deal with them.