From that perspective, any machine or tool will have to be rendered obsolete, and frankly, that includes everything, starting from basic machine tooling to a blacksmith's hammer or a casting mold. This phenomenon is called "dual use", and if it gets too common, it just loses it's purpose. Also, restricting certain products will only cause market to figure out a method around it.

Pressure-bearing parts are not 3D printed, never, unless you've got a laser sintering machine at your disposal, which currently cost +100k and beyond, and the materials, planetary ball milled metallic powders, that ignite upon contact with atmospheric oxygen, cost $300/kg and above.

Also, there are countless items that share similar mechanisms. If you know gun mechanics, you know how universal shapes those are. For common people, you only need 1-2 seconds with a gun to disassemble it into two or more parts to make it unrecognizable as a firearm to most of them, and that's only with guns that look like guns.

This is not black and white scenario. This is a scenario with technology that supports other technologies. While making pressure bearing parts from metal is easy-ish with basic machine tooling, frames, receivers, grips, stocks and many secondary parts isn't - but then you can 3D print them. Now you have a gun that has all important - but simple - parts made out of high strength alloys, but all secondary parts 3D printed.