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.
The first question presented to the ATF was
You transfer an AOW firearm that you manufacture to a customer on an ATF Form 4 and he takes it home. The customer then orders a 20 inch 12 gauge shotgun barrel, magazine tube and shoulder stock. He then removes the 10 inch barrel and pistol grip, and installs the 20 inch barrel, magazine tube and shoulder stock. He has reconfigured the AOW into a sporting firearm; can the owner use it for hunting or target shooting?
The ATF determined that this was legally permissible and stated that Federal statutes do not prohibit the reconfiguring of a legally registered AOW firearm into a sporting firearm and then back into an AOW firearm by the owner.
NOTE that the ATF underlined the words by the owner.
The ATF was also asked:
Can a police department that has purchased a 7. 5 inch breaching gun change the configuration by installing a 20 inch barrel and shoulder stock/or a different mission and then reconfigure it back into an AOW?
and the ATF stated that State and local police departments that have a legally registered AOW
can reconfigure the AOW in any manner the mission requires.