RE: Legal Barrel Length
I believe a shotgun barrel must be MORE than 18 " in length and the OVERALL lenght of any shotgun must be MORE than 26 ". Otherwise, any such weapon can be deemed a "short-barreled shotgun" and is subject to a violation. I think that is the federal statute you're talking about.
Have you checked with Gun E-Parts (the Numrich people) I bought a pretty short barrel for my 20 ga Mossberg years ago (about 18 and 1/4 inch I think) with rifle sights installed. It is just a re-claimed cosmetic flaw barrel that they bought in mass quantities , sawed off to a legal lenght, added the sights, and sold for $50.00. That was about 15 yrs ago.