It is insane that if you put a shoulder stock on a pistol that it instantly becomes a “short barreled rifle” that is subject to unconstitutional NFA rules. The fact that SBRs, sawed off shotguns, and suppressors are subject to the NFA is stupid, too.
The reason that SBSs and SBRs are on the NFA list is that the original intent of the legislation was to BAN ALL PISTOLS!
Therefore, they also wanted to ban anything that could be “converted” in to a pistol as well.
They found no public support for banning pistols, so that part was dropped. The SBR/SBS ban remained, however.