Originally Posted by nanewt02
...since it only fires 12 ga black powder ammo
But that's not true. It might be smart to fire only black powder shells. However, the gun will chamber modern, smokeless powder shells and fire them -- until it blows up. And with lightly loaded smokeless powder ammunition, and if the barrels turn out to be high quality and in excellent condition, it may stand up to such use for some time. I've read, in articles on vintage shotgunning, that some folks do
use light smokeless powder loads in Damascus barrels.
I sure wouldn't do something like that, and I think that BigDinFL has the right idea. But it is still incorrect to say that a shotgun with Damascus barrels can only fire black powder shells.
In any event, under the National Firearms Act, an antique fire arm is defined as (26 USC 5845(g)) as (emphasis added):
(g) Antique firearm
The term “antique firearm” means any firearm not designed or redesigned for using rim fire or conventional center fire ignition with fixed ammunition and manufactured in or before 1898 (including any matchlock, flintlock, percussion cap, or similar type of ignition system or replica thereof, whether actually manufactured before or after the year 1898) and also any firearm using fixed ammunition manufactured in or before 1898, for which ammunition is no longer manufactured in the United States and is not readily available in the ordinary channels of commercial trade.
However, as mentioned above, your 12 gauge, Damascus barrel shotgun will chamber and fire standard, modern shotgun shells (although doing so probably isn't a good idea). And black powder shotgun shells are commercially available
So since your shotgun would not be an antique under the NFA, it wouldn't be exempt from regulation under the NFA if you cut down the barrels.