Anyone with a decent machine shop (lathe and milling machine) and relatively modest machining skills can already make a gun any time they want to. A shotgun would be relative child's play to make. Rifling a barrel takes a bit more technical knowledge. But heck, any machinist that has a gun, can disassemble it and make all the parts necessary to create more. With CNC machinery, he could be in production for mass volumes rather quickly.
The technology of making guns is a cat that has long been out of the bag. No one can EVER get that cat back into the bag.
But certainly these 3D printing devices are going to be an incredible advancement in technology to watch as it matures and broadens in scope.