The saddest thing about regulators is often a small payment to the regulated would be more effective and less costly in achieving the same desired result.
They should simply pay people to register 3D printed guns, up to a specific amount, at which point: they should investigate them for illegally manufacturing guns.
Similarly, they should severely penalize possession of a 3D printed gun which has not been registered.
Problem solved. Good luck pretending these people are capable of regulating the compliance of 3D printing software.
What a joke.