To be fair, it's only true that people are making firearms on 3d printers because what we consider the firearm in the US is unusual. In other places the portion

To be fair, it's only true that people are making firearms on 3d printers because what we consider the firearm in the US is unusual. In other places the portion of the machine that contains pressure is the firearm. In the US the portion of the machine that holds the trigger mechanism is considered the firearm. This leads to the easiest part of the machine to manufacture also being the only part that is illegal to manufacture in places where such a thing is illegal. It's exactly backwards from physics. Making a 3d printed firing chamber is exceedingly difficult, especially one that won't blow your hand off. Making a trigger - even a fully automatic one - is one notch above trivial.