TexasRifleman
Moderator Emeritus
this was addressed in United States v. Thompson Center Arms Co. (1992). In TC, the Supreme Court held that a TC Contender pistol (accompanied by a rifle length barrel and butt-stock) was not an SBR via constructive posession, that there was a lawful purpose to posessing the buttstock (it could legally be attached after the coposessed rifle barrel was installed).
Careful with Thompson. The ATF has interpreted their loss to apply only to a specific subset of TC firearms sold as a complete kit with both rifle and handgun parts.
ATF has said that a Thompson purchased as a handgun or rifle only , and the other pieces purchased later, does not fit into the ruling.
I think they are out of their minds, but they are on record with that position.