short barreled shotguns do not nessesarly fall under AOW. They only fall under if they only have a pistol grip butstock and have never had a regular buttstock. Also they can not have a rifled bore (mute point who wants a AOW 12 in slug gun).
The tax is $200 for a short barrel shotgun and you will have to have the approval of the chief law enforcement officer in your area. If you are in Kalifornia and not a LEO, forget it.
14" rifle sighted barrels are available directly from Remington.
Scattergun Tech features <18" barrels on some of their guns. I don't know if they sell barrels separately, but it can't hurt to ask. http://www.scattergun.com