By federal definition, a firearm manufactured prior to 1898 or any replica of any firearm if it is not designed or redesigned for using rimfire or conventiaonal centerfire fixed ammunition, or if it uses rimfire or conventional centerfire fixed ammunition which is no longer manufacturered in the US or is not readily available.
Also, andy muzzle loading rifle, muzzle loading shotgun, or muzzle loading pistol which is designed to use blaock powder or plack powder substitude and which cannot use fixed ammunition.
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