Shoulder fired and capable of firing a fixed shotgun shell makes it a shotgun by definition, virgin receiver or not, rifled barrel or not.
A rifled barrel would not make it an SBR because of the bore diameter being >.50.
To answer the OP's specific question, with a rifled barrel, it is still a shotgun...
so i couldn't SBR a .600 nitro express?
Darn. Now I really want to.