We recently received a copy of a letter dated November 2, 2010 that addressed this issue. The person wanted to know if they had manufactured an AOW with various-length barrels, could a stock and longer barrel be added later for the purpose of hunting or other legal uses.
The ATF first responded with As background, the National Firearms Act (NFA), 26 U.S.C. Section 5845(e), which defines “any other weapon” to include-
… Any weapon or device capable of being concealed on the person from which a shot can be discharged through the energy of an explosive, a pistol or revolver having a barrel with a smooth bore designed or redesigned to fire a fixed shotgun shell, weapons with combination shotgun and rifle barrels 12 inches or more, less than 18 inches in length, from which only a single discharge can be made from either barrel without manual reloading, and shall include any such weapon which may be readily restored to fire . Such term shall not include a pistol or a revolver having a rifled bore, or rifled bores, or weapons designed, made, or intended to be fired from the shoulder and not capable of firing fixed ammunition.