18 USC Sec. 921 (a-3): "The term "firearm" ... does not include an antique firearm."
18 USC Sec. 921 (a-16): "The term "antique firearm" means -
(A) any firearm ... 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, or (ii) 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; 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...."
(source)