Any muzzleloader than can be converted with ease to a cartridge gun via a barrel change require an FFL. The Remington 700 ML is one of these, along with the Traditions NitroFire Muzzleloader and several Thompson Centers. Here is the ATF Ruling (2005)
MUZZLE LOADING FIREARMS
We frequently receive questions concerning the sale of modern in-line muzzle loading rifles. Several years ago ATF determined that an in-line muzzle loading rifle using #209 shotgun primers for ignition was not an antique firearm and was subject to all provisions of the Gun Control Act (GCA). However, the GCA was amended in 1998 and many in-line muzzle loading rifles are now antique firearms and are excluded from the provisions of the GCA.
As defined in section 921(a)(16) of Title 18, U.S.C. the term “antique firearm” means –
(A) any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898; or
(B)any replica of any firearm described in subparagraph (A) if such replica —
(i)is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition,(ii) uses rimfire or conventional centerfirefixed ammunition which is no longer manufactured in the United States and which is not readily available in the ordinary channels of commercial trade; or
(C) any muzzle loading rifle, muzzle loading shotgun, or muzzle loading pistol, which is designed to use black powder, or a black powder substitute, and which cannot use fixed ammunition. Forpurposesofthis subparagraph, the term “antique firearm” shall not include any weapon which incorporates a firearm frame or receiver, any firearm which is converted into a muzzle loading weapon, or any muzzle loading weapon which can be readily converted to fire fixed ammunition by replacing the barrel, bolt, breechblock, or any combination thereof.
Many modern in-line muzzleloaders including those that use #209 shotgun primers for ignition now meet the definition of “antique firearm” and are excluded from the provisions of the GCA. However, there are some muzzleloaders that are still firearms subject to GCA controls. For example, firearms which can be switched from muzzleloaders to breechloaders by exchanging the barrel are still GCA firearms. In addition, a modern firearm that is altered to function as a muzzleloader is still a firearm.
The following are some other examples of muzzleloaders that are still GCA firearms:
Thompson Center G2 Contender 209-45 Thompson Center Encore 209-50
The H&R 1871 Huntsman; Rossi S50; Mauser 98 fitted with a muzzle loading barrel Remington 870 fitted with a muzzle loading barrel; Mossberg 500 fitted with a muzzle loading barrel
To the group
