For some reason the link you posted keeps bringing me back to this same thread.
All the hype about 3D printed guns is quite funny when you can make or finish a really nice lower receiver with an inexpensive CNC milling machine. Of course it is cheaper and probably easier in most cases to do it starting with a partially completed lower with jigs and common tools.
See post #13 re the Raich and Stewart decisions. According to established case law, the Commerce Clause permits Congress to criminalize 3D printed guns for private ownership.