Fully 3D-printed guns have awful performance, though. You would likely get significantly better results improvising with the stuff available at literally any random hardware store. Yet, despite that kind of gun being around for centuries, we weirdly aren't seeing bans on rigid metal tubes yet.

3D-printed "guns" become a real issue when you combine it with unregulated sale of firearm parts and ammunition. To get a fully-functional gun you just need to 3D print a fairly trivial component which is legally considered the entire gun as it carries the serial number. But that's not a 3D printing problem, because there are also companies selling that same part in a mostly-finished legally-not-a-gun form, together with a drilling jig guiding you how to drill the last few holes with a regular Dremel. And nobody is proposing banning Dremels. Heck, it is totally okay to own a lathe - which you can use to make your own high-quality guns!

And the entire discussion is of course pointless once you realize that this is the USA, so anyone is only a weekend road trip away from legally and fully-anonymously buying a gun two states over. If 3D-printed guns are such a huge problem, why aren't we seeing European countries mass-banning 3D printers?

To extend your analogy: it's like being fine with the sale of ultracentrifuges and uranium hexafluoride, then getting upset at someone selling a screwdriver to attach the plug to the power cord of the ultracentrifuge because "screwdrivers lead to nuclear bombs".

3D printed guns are a nothingburger. There is indeed a non-zero number of violent crimes committed with them - but there is also a non-zero number of violent crimes committed with shoelaces, so that's clearly not enough of a reason to ban them. It only makes sense to regulate them if they are involved in a significant number of crimes and leading to a huge increase in gun violence - and at that point you probably want to crack down on all forms of DIY guns instead of just the 3D printed ones. But that's simply not the case, so the regulation is pointless and doing more harm than good.