As soon as somebody produces a "pistol" upper for .50BMG - it instantly classifies that caliber as a pistol round.
There are all sorts of legal ramifications to this - AP ammo (armor piercing) for instance instantly becomes illegal to own for civilians.
This is why you cannot get .223 and .308 AP ammo for civilians - because some dimwit produced a pistol in those calibers. It is why 30.06 AP is still legal for civilians to own - because nobody (yet) has produced a pistol in that caliber.
This isn't completely true. Things like M855, and M2 ball AP have line item exemptions on them, I think. The 50 is definitely vulnerable, though, to what you describe, at least if a pistol was -commercially produced- eg, a fed manufacturer made it or "offered it for
sale."
If someone makes one in their basement, and doesn't offer it for sale, then it doesn't matter and doesn't trigger the determination change.
-Mike