I think that is spelled out too (in the gun control act of 1968). If it is a sawed off shotgun or a machine gun, modern rules apply. I.e. it is considered same as a modern gun in that case.

I do want to point out that I AM NOT a lawyer( which is obvious to the fine legal mind of Rookhawk) and if you are contemplating buying anything questionable, get a NRA or otherwise gun savy lawyer to tell you what is legal and what is not.