[azernik]

See https://news.ycombinator.com/item?id=25514950

tl;dr - you don't "have to register" them, because by definition the ones that are legal are already registered. In '86 they stopped allowing new registrations for automatic weapons, but existing registered weapons were grandfathered in. US gun law considers the lower receiver to bear the "identity" of the weapon, so some pretty extensive modification can occur while it technically stays "the same gun" in a Ship of Theseus fashion.