I recall that some of the designers claimed (not legal advice) that no law prevented anybody from building his or her own gun, because it doesn't affect interstate commerce. However, it is illegal to sell the gun in question, because it would then enter interstate commerce. I also recall a statement to the effect that even a privately-built gun has to have enough metal to make it detectable by security equipment.

I have to object to one point you made regarding infringement. The threat of infringement of our second amendment rights to gun ownership is real, and not just some right-wing hysterical conspiracy theory. The many very intrusive arms of our now left-leaning government and its similarly left-leaning employees are posturing our system for just that: infringement of our gun rights. Given the two thirds majority they seek, you can rest assured that the second amendment would undoubtedly either be repealed, or altered beyond recognition, effectually neutering our rights as gun owners--and then what beyond that. I agree that such 3D printing technology to create weaponry from one's garage or basement, to any sane person, is dangerous to our personal and national security. But let's keep this conversation honest, with both eyes wide open. Easily accessible plastic guns should not be made into the justification to erode our constitutional freedoms.