bmg50cal
Regular Member
26 U.S.C. § 5845(E)
For the purposes of the National Firearms Act, the term “Any Other Weapon” means:
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
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.
http://www.atf.gov/firearms/guides/importation-verification/firearms-verification-nfa-aow.html
According the manufacturer it is considered an "AOW pump shotgun." When it was redesigned it was neither a pistol nor a revolver to begin with so the second USC definition does not apply. The first applies.