@icedquinn @arh@toots.alirezahayati.com the lower receiver has always been "legally a gun". buying a lower receiver, even if not fitted out with trigger, upper, whatever, is still "buying a firearm" for legal purposes.

you have to buy a lower that isn't completed, which is uncontrolled, and do a bit of aluminum milling yourself, then add the upper and trigger parts (also all uncontrolled). it's still common and legal to do so.