If a firearm was made AS A HANDGUN- and is smoothbore, less than 18 inch bbl, it would be an AOW. $5 transfer tax
If it started life as a shotgun, barrel over 18', shoulder stock, etc, and was modified to be less than 18", it is an SBS. $200 tax
Please note that SOME high capacity shotguns that have a bore diameter greater than .50 are not SBS's nor AOWs- they are Destructive Devices (DD's) such as the StreetSweeper, Striker 12, etc. $200 tax and illegal in many states whether you have Federal paper on it or not.