3D printers.
Yes, 3D printers.
Of course, when the first 3D printed gun was made, it wasn't a very high quality firearm, which proved there will never be any high quality 3D printers. Just like Gutenberg's printing press is not an ancestor of the internet. And the very first film movie camera is not in any way connected to a DVD.
Gun Control is actually a lost cause.
Yup. By the time we could get all the hundreds of millions of guns out of circulation 3D printing will reach the point that it can make high quality guns. There's no point in embarking on a mission whose failure is certain.