If you bought a shotgun frame that someone MIGHT have had a too-short barrel on at one time, and you put a legal length barrel on it- it would be legal.
I too kind of doubt that is always true. In the case of an Ithaca Stakeout, I found out that I can't even add a buttstock to it, dispite the previous owner selling it under a picture of it with both a full buttstock and a pistol grip. Course he also shipped it to a class III dealer from out of state (thinking his ffl was enough)and didn't go thru another class III dealer.The funny part was the transfer was approved like that! My dealer walked out to the car when I drove up, and handed me a buttstock and said when I got "rid" of that, come on in and we would finish up the paperwork. Adding a longer barrel to that receiver would tend to make me wonder if it were legal to carry across state lines without having to apply for the proper paperwork. No basis for my doubt, but I still doubt it!
David from jax
On Edit, the Stakeout is an Ithaca shotgun with a 12" barrel and pistol grip, thereby making it an AOW, not a SBS.