Downloadable 3D gun plans have been ubiquitous online for about five years. Almost nothing has changed in reality. Anyone tech savvy enough to 3D print a gun is certainly capable of accessing the Pirate Bay.
Infringing on what is a clear exercise of first amendment rights isn't going to change anything, and it would be a horrible precedent to set. Whatever your feelings on gun control are, the ability of the government to control the physical items in the possession of the populace is going to be over soon. Just like ride-sharing, Air BnB, and a bunch of other new tech, 3D printing is going to leave the government increasingly out of the picture and put the power to create where it belongs, in the hands of people. Without government or corporate gatekeepers.
Like the war on drugs, the war on 3D printing will be lost. Give it up. Whatever you do, however you try to interfere, people are going to have the ability to make what they want, by themselves, in their basements. More and more cheaply, more and more easily. People, working class people, all of us, will have access to technology that can make the stuff in our heads into material fact.
If you don't think this is good, please, don't call yourself a liberal. You're not. Either way, though, like it or not, it's going to happen. You can't stop the democratising force of technology.