part of the problem with that is that most of your ghost guns are “just” the AR 15 lower receiver- the part classified as the “firearm” because of some technicality only a lawyer can understand.

It’s not a load bearing, nor is it exposed to any particularly high levels of heat, nor any sort of particularly nasty gasses. So that can be printed in just about anything if the printer’s resolution is good enough. (IIRC, they typically call for resin printers.)

Then, the ghost gun peeps just buy the rest of the components retail and pay cash.

for fully-printed firearms, you’re looking at things like DMLS or other kinds of precision metalwork. It’s the kind of work that would be more expensive than roadtripping to arizona and getting loophole-gun.