It's a question I've wondered about many times.
The Federal definition of an antique is "made before 1899 or does not use fixed ammunition". There's some other parts, but that will do for this. The "does not used fixed ammunition" part gets around NFA. There were, some 20 or so years back, a bunch of percussion SxS shotguns with about 13 inch barrels. They were advertised as "Confederate Cavalry" guns. ATF had no problem with them. There is currently being made a Howdah pistol in both 56 caliber rifled and 20 gauge smoothbore. If that were fixed cartridge it would certainly be NFA, but it's percussion.
And we can't forget the Lematt pistol, with it's shotgun center-pin. Definitely a short barreled shotgun, but it's percussion, so it's okay.
Since part of the "antique gun" thing is exempt from NFA, I would think that all of it would be.
But before I risked my liberty and bank account, I'd write ATF and get an answer on ATF letterhead. Write
, not email or telephone. That way you have a physical document to prove what you were told.
Meddle not in the affairs of dragons, for thou art crunchy, and taste good with catsup - George of Lod, Year of Our Lord 297
I always take precautions.
Beware the Evil Bullet Fairies.