There are SBS Mossbergs available at retail.
Not sure if they're purchased from Mossberg pre-configured as an SBS or if the retailer "manufactures" them, but they're out there. Buy one, submit your Form 4, pay your $200 tax stamp, and in 6-8 months it could be yours.
Itsricmo,
A shotgun is classified as an AOW only if it was originally manufactured in a pistol grip only configuration with a short barrel or had an 18" or longer barrel that was later shortened. The reason this is classified as an AOW is because there is no classification for a smoothe bore pistol with a bore diameter greater than .50". A shotgun purchased with a stock, reconfigured with a pistol grip only, and barrel shortened to under 18" is an SBS, not an AOW.
Having a shoulder stock is part of what classifies a rife as a rifle. For this reason, there is no such thing as an AOW rifle. It's either a rifle or an SBR. The closest you'll get in concept is an AR pistol. It must be built from a virgin receiver that has NEVER had a stock attached and must have a pistol buffer tube not capable of accepting a stock. There are .22lr versions that don't need a buffer tube and use a receiver end cap instead. Once you are in posession of this lower receiver set-up, you are legally allowed to purchase and mount an upper with a barrel under 16". According to the BATFE, this configuration is legally classified as a pistol. Of course some states differ in what is allowed, so it's always a good idea to contact your local BATFE office for laws in your area...