So did some checking on the definition. If it has a receiver or frame and removable barrel, its classed as a firearm, not muzzleloader.
Quote:
The ATF has previously determined that certain muzzle loading models are firearms and subject to the provisions of the Gun Control Act of 1968 (GCA).
All of these guns incorporate the frame or receiver of a firearm that is capable of accepting other barrels designed to fire conventional rimfire or centerfire fixed ammunition.
Therefore, these muzzle loading models do not meet the definition of “antique firearm” as that term is defined in the above-cited § 921(a)(16) and are “firearms” as defined in 18 U.S.C. § 921(a)(3)
Furthermore, as firearms, the models described above, as well as other similar models, regardless of installed barrel type, are subject to all provisions of the GCA.
The following is a list of weapons that load from the muzzle and remain classified as firearms, not antiques, under the purview of the GCA since they incorporate the frame or receiver of a firearm:
-Savage Model 10ML (early, 1st version).
-Mossberg 500 shotgun with muzzle loading barrel.
-Remington 870 shotgun with muzzle loading barrel.
-Mauser 98 rifle with muzzle loading barrel.
-SKS rifle with muzzle loading barrel
-RPB sM10 pistol with muzzle loading barrel.
-H&R/New England Firearm Huntsman.
-Thompson Center Encore/Contender.
-Rossi .50 muzzle loading rifle.
This list is not complete and it frequently changes; therefore, there may be other muzzle loaders also classified as firearms. As noted, any muzzleloader weapon that is built on a firearm frame or receiver falls within the definition of a firearm.