As usual, I have to wonder first if anyone actually put any thought into this, and further if anyone thought how the fuck they're going to enforce it. This is just manufacturing one step removed... Anyone willing to make a gun with a 3D printer is certainly capable and willing of building their own 3D printer as well.
Or buying/building a milling machine. Or a lathe. Or a drill, a hacksaw, and some files.
Down this road lies madness.