Any Other Weapon (AOW) - Any National Firearms Act (NFA) firearm not otherwise categorized by the Bureau of Alcohol, Tobacco, Firearms, & Explosives (BATFE) as a short-barreled rifle, modern shoulder-stocked pistol, short-barreled shotgun, machine gun, sound suppressor (silencer), or destructive device.  The Federal Transfer Tax on any AOW is $5 (lowered from the original $200 tax in 1960).  If you intend to build your own AOW or convert an existing firearm into an AOW, the 'making' tax of $200 is paid in lieu of the $5 transfer tax.

Included in the list of AOWs are firearms firing conventional fixed ammunition, and disguised so they are not readily recognizable as firearms, such as pen guns, cane guns, and wallet holsters that hide the outline of the firearm and allow it to be fired while holstered. 

Also included are conventional fixed ammunition firing pistols that were originally equipped with both front and rear pistol grips.  Although ATF also considers the addition of an aftermarket foregrip to be making a AOW, this action is not supported by law.  At least two cases brought by the government have been won by the defendants, but ATF still enforces this policy, so caveat emptor! 

Perhaps the most common classification of AOWs is the smoothbore handgun.  Conforming to Congress's description of a "concealable" firearm, ATF considers a firearm to be an AOW if it has a smoothbore, fires conventional fixed ammunition (including shotgun shells), was originally equipped with a pistol grip, has never been fitted with a shoulder stock, and has an overall length less than 26".  This should not be confused with handguns that use rifled bores, and fire conventional, fixed ammunition less than .50 caliber.  For example, the Taurus Judge can fire a .410 shell, but since it is less than..50 caliber and the bore is rifled, it is not considered an AOW.  Nor should an AOW be confused with a smoothbore handgun over 26" in length, which ATF categorizes as a "firearm".  Examples include the Mossberg Cruiser with pistol grip only (PGO), and the Remington 870 PGO.  Because they nominally are handguns, and not shotguns, these PGO smoothbores can be registered as AOWs and shortened, provided they didn't come with an optional shoulder stock, and no shoulder stock was ever fitted to them.

One last category of AOW is the combination rifle/shotgun, where the barrels were either originally made or later shortened to less than 18", but no shorter than 12", and can only be manually loaded and reloaded for each shot.  This type of AOW is allowed to have a shoulder stock. 

Two common examples of "Any Other Weapon" (AOW), a five-round magazine Mossberg 500 Cruiser with a 15.5" barrel in .410 bore (left), and a 14" M6 Scout combination gun in .22 Hornet/.410 bore (right)

THEN WHAT ARE THESE?

According to ATF, neither of these guns is classified as an AOW or SBS.   They are "firearms", and are treated as conventional handguns because they meet two legal conditions required by ATF: their overall length exceeds 26", so they are not deemed "readily concealable" as defined by Congress; they were originally provided by their manufacturer with only a pistol grip and have never been fitted with a shoulder stock.  If either of these conditions changes, their status changes to either shotgun, AOW, or SBS.

Since the gun on the left has an 18" barrel, fitting it with a shoulder stock simply turns it into a legal shotgun, so no ATF paperwork is required.  But the gun on the right has a 14" barrel, so arbitrarily fitting it with a shorter grip turns it into an unregistered AOW since it's overall length now will be less than 26".  Fitting it with a shoulder stock turns it into an unregistered SBS because its barrel is less than 18".  This is the reason why all such smooth bore handguns that are made by companies such as Mossberg and Remington, are fitted with 18" barrels.  It's done to keep you from accidentally entering into a legal nightmare, should you ever decide to customize your firearm.

AND WHAT ARE THESE?

The two guns pictured above are both AOWs because they were made from smoothbore SxS firearms that never had shoulder stocks, and therefore were not originally classified as shotguns.  On these guns the barrel lengths are immaterial, but since their overall lengths are under 26", they are classified as AOWs.  All AOWs are subject to a $5 transfer tax if you are acquiring one from somebody else, or a $200 "making" tax if you convert or build a gun yourself.  However, the availability of SxS and O/U AOWs is extremely limited.  The reasons for this are threefold.  When the National Firearms Act of 1934 was enacted, it placed the same $200 transfer tax on AOWs as it did on short barrel shotguns (SBS).  For the few people back then who could afford the tax and desired such a gun, it made more sense to acquire a SBS since one could add or remove a shoulder stock from it at will.  For example, production and sale of the original Ithaca Auto & Burglar SxS smoothbore pistol (top photo) was so adversely affected by the original $200 transfer tax, that only about 4,000 were ever built, mostly in 20 gauge.  A handful of 410, 28, and 16 gauge also were made.

By the time Congress revised the law and reduced the transfer tax to $5 on AOWs in the early 1960s, public interest in SxS and O/U shotguns had waned in favor of pump and semi auto guns.  Few, if any, manufacturers were interested in selling SxS or O/U AOWs, virgin receivers, or unstocked guns.  By 1990 this became a moot point as domestic production of affordable O/U and SxS shotguns had ceased in the USA. 

Around 1999-2000, one company, sensing a niche market for an updated version of the Ithaca, began offering the "Denny's Guns Auto & Burglar" (bottom photo).   Purportedly, these were made from imported Italian shotgun frames that lacked shoulder stocks, and therefore were never classified as shotguns.  The number built is unknown, but reputed to be fewer than 100.  Available chamberings are believed to be 410, 20, and 12 gauge.  Until some enterprising manufacturer or importer picks up the mantle, the availability of new SxS or O/U AOWs will remain moribund.

This is a short barrel shotgun because it was converted from a shoulder stocked smoothbore firearm (shotgun).  Any shotgun measuring less than 26" in OAL, or having a barrel less than 18", is classified as a short barrel shotgun and subject to the $200 transfer or making tax.  Affordable, domestically produced SxS shotguns have not been available since the late 1980s, but are viable candidates when suitable models can be located at reasonable prices.  Some second-hand Italian, Spanish, and east European guns also are available, but quality can vary from horrid to very good, so caveat emptor!  If a new modern SxS shotgun is desired to form the basis for a conversion, the most common budget priced guns are Russian, Brazilian, Turkish, and Chinese imports.  The quality of these guns runs from adequate to good, but they are not on par with classic English or western European guns.  The latter types retain such high values that converting them is not recommended, as it would destroy their worth.

What does it all mean? 

It means the only type of smoothbore AOW you are likely to find or be able to make yourself is a pump-action version.  But be careful when making your chocie.  A Class 2 manufacturer can turn a gun such as a Mossberg Cruiser (comes with pistol grip only) into an AOW by shortening the gun to less than 26".  The buyer pays the $5 tax when purchasing it.  But the manufacturer cannot turn any shoulder-stocked shotgun or even a kit gun (a shotgun that comes with both a pistol grip and shoulder stock) into an AOW.  Such guns, if shortened to less than 26" overall or fitted with a barrel under 18", must be registered as a SBS, which means the $200 transfer tax must be paid by the buyer.  The advantage of the SBS is that it can be configured with either a shoulder stock or pistol grip.  With the exceptions stated in the paragraph above, most AOWs can have only a pistol grip - not a shoulder stock.  If you own a pistol gripped smoothbore firearm, make sure you keep the original box it came in.  That may be your only proof of its origin, should you decide to have it converted into an AOW.