weapons over 50 cal (with an exception of sporting weapons)
Short brl rifles (brl less than 16)
Short brl shotguns (brl less than 18)
machine guns (fires more than once for each pull of the trigger)
any other weapons (weapon that do not really fall into other catagories like MP-5K (assuming its a semi) or pen guns)
So if you ask the atf if a weapon that is no longer really a shotgun at heart can a brl two inches shorter I think they would ask you to register it as a weapon over 50 cal or a SBS.
Also I belive that if a weapon is registered as one thing (say a mg) it does not need to be registered as another thing (say sbr)... Except weapons registered after 1986 can not be machine guns because the Gun Owner Protection Act made it illegal to register anymore mg.