I actually wouldn't disagree that it shouldn't be legal to 3D print a gun, that does seem like something seriously difficult to actually regulate though. If someone's 3D printing a gun on their private property behind closed doors for example, how would you catch them in the act with that? From what I've heard before there's actually another thing aside from ammunition that you can't really print, and that's the firing pin. Apparently you can 3D print the firing pin, but it fails after a handful of shots.
There's actually a lot of things with the advancement of technology that are quite terrifying. Take the development of AI for example, if the AI doesn't decide to kill us, if we can actually keep it under control then what sort of people are going to be the ones in control of it? Even the scenario where humans can control AI is still quite horrifying when you think about it, and that kinda makes me think that humans playing with AI is like letting a drunk 4 year old play with a box of hand grenades at a gas station lol.