In US, the receiver of the firearm is legally the firearm -- everything else is just boring bits of metal that you can buy without any sort of legal encumberments. Anyone can buy a chunk of 7075 aluminium that looks like an AR-15 lower receiver but, critically, has enough milling not-yet-done such that the ATF has blessed it to not be a firearm. Some time with a proper jig and a drill press can convert it into a working lower receiver! You don't even need a CNC.

But you don't even to buy such an "80% lower receiver" to do this -- you can just make your own lower receiver out of most anything (it need not stand firearm chamber pressures, unlike the bolt carrier / bolt and barrel, which are much harder to DIY), up to and including wood.

Murders done with rifles are quite rare (an overwhelming majority of murder is done with handguns, per FBI's Uniform Crime Report) so the disproportionate focus on the evils of such "ghost guns" / "DIY rifles" is baffling.