Guns require parts made of quality steel in order to work effectively. Parts like the barrel, bolt, receiver, and other parts that are subjected to high pressures cannot be “printed” at this time, and it will probably be a while before that kind of technology is perfected, as these kinds of parts are usually machined with extremely tight tolerances. In short, you cannot make a complete printed plastic or polymer gun and expect it to work well, if at all, but that probably won’t stop people from freaking the fuck out about this and going into ultimate hand-wringing mode, as I’ve seen in comments sections on articles about this topic recently.
Also, if a ton of people started printing things like AR-15 lower receivers (which is completely legal) and whatnot, the ATF would probably start restricting barrels, bolts, and other parts that can’t be effectively printed at this time. If it were feasible to print these parts, then any gun control effort would be null and void unless there was a push to ban 3D printers or some other reactionary shit.
Also, it is important to note that this isn’t exactly a new development. You can do the same thing with a CNC machine and have a gun made out of actual metal. Additionally, you can’t “print” ammo and that’s probably never going to be possible. Also, 3D printers are still extremely expensive.
So yeah. People shouldn’t panic about this.