The specific definition from the Gun Control Act of 1968:

18 U.S.C., § 921(A)(16)

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; and is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition, or

B. Any replica of any firearm described in subparagraph (A) if such replica  uses rimfire or conventional centerfire fixed ammunition which is no longer manufactured in the United States and which is not readily available in the ordinary channels of commercial trade.

C.  Any muzzle loading rifle, muzzle loading shotgun, or muzzle loading pistol, which is designed to use black powder, or black powder substitute, and which cannot use fixed ammunition. For purposes of this subparagraph, the term antique firearm shall not include any weapon which includes 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.