BATFE's position was formalized in April 2008 in their NFA Handbook (ATF Publication 5320.8 April 2008)[7] which classifies an AOW as follows:
2.1.5 Any other weapon.
Firearms meeting the definition of "any other weapon" are weapons or devices capable of being concealed on the person from which a shot can be discharged through the energy of an explosive. Many "any other weapons" are disguised devices such as penguns, cigarette lighter guns, knife guns, cane guns and umbrella guns. Also included in the "any other weapon" definition are pistols and revolvers having smooth bore barrels designed or redesigned to fire a fixed shotgun shell. While the above weapons are similar in appearance to weapons made from shotguns, they were originally manufactured in the illustrated configuration and are not modified from existing shotguns. As a result, these weapons do not fit within the definition of shotgun or weapons made from a shotgun. The "any other weapon" definition also includes specifically described weapons with combination shotgun and rifle barrels 12 inches or more but less than 18 inches in length from which only a single discharge can be made from either barrel without manual reloading. The firearm most commonly associated with this portion of the definition is the Marble's Game Getter. The "any other weapon" definition excludes weapons designed to be fired from the shoulder that are not capable of firing fixed ammunition or a pistol or revolver having a rifled bore. However, certain alterations to a pistol or revolver, such as the addition of a second vertical handgrip, create a weapon that no longer meets the definition of pistol or revolver. A pistol or revolver modified as described is an "any other weapon" subject to the NFA because the weapon is not designed to be fired when held in one hand. As stated above, a pistol or revolver having a rifled bore does not meet the definition of "any other weapon" and is not subject to the NFA. It is important to note that any pistol or revolver having a barrel without a rifled bore does not fit within the exclusion and is an "any other weapon" subject to the NFA.