Handguns must have rifled bores or they would need to be registered as AOW under the regulations of NFA.
ANY OTHER WEAPONS
An AOW is:
"...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 smooth
bore designed or redesigned to fire a fixed shotgun shell,
weapons with combination shotgun and rifle barrels 12" or
more, less than 18" in length, from which only a single
discharge can be made from either barrel without manual
reloading, and shall include any weapon which may be readily
restored to fire. Such term shall not include a pistol or
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." 26 U.S.C. sec.
5845(e).