I promise you all one thing: The moment 3D-printers are somehow regulated what it can be used for all the lobbyists will come forward and will add their wishes to the list of forbidden things to print.
If 3d-printing is outlawed so the chance of getting the power to manufacture things and spare parts will end. All your prototypes will go to a central authority to see if there is something useful in development... Or to see if there are copyright violations...
And no, it will not stop ghostguns. This ship sailed a long time ago and it will not come back. Society needs other ways to handle it than forbid a manufacturing process.