Here is the statement in the law. The term here is "uses", not "was intended for" or "might chamber" or "is marked for". The assumption would probably be that if the cartridge fits and fires safely in that gun that the gun "uses" that cartridge.
(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
(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.
Hi, Matt,
It might help to know the reason for the question. Are you planning to convert an antique Sharps, or manufacture reproductions, or is the concern with shipping, etc.?
Jim