Stroke of a pen, it would seem.... suddenly, you are required to have a state-issued license to manufacture/print any type of firearm.
Problem solved.
Yes, I understand terrorists and felons aren't going to get a license.
Aren't all printers made with an embedded mark that identifies the make and model? Been like that for years.
Do the same with 3-D printers.
Five minutes after a manufacturer puts out something that adds an embedded mark, someone will find the means to remove it and spread that information on the net like wildfire.
At that, for regular printers, if you google it, you can find hundreds of sites and videos on how to remove those identifiers in minutes.
Government prohibitions never work. They actually bring more attention to the information. It is like attempts to ban books, they become more popular. I doubt anyone would have hear of the "Anarchist Cookbook" if it was just a few guys passing around offset printed pages made in their garage. But all the attempts by the government to ban it have created a market where millions (if not more) copies have been passed around and downloaded. (I have a whole library on a flash drive of 'banned' information like this).