There’s a 3D printer for that already.
For full auto? I know that there's a program for an 80% receiver (plus the printer, of course).
Personally, I'd like it if it was at least nominally legal to own, since I'd rather avoid a SWAT raid on my home. Yes, I know, the law itself is illegal per the 2nd Amendment, but I don't have SWAT teams at my beck and call.