A "gun" can be made 100% at your local hardware store, using mass produced parts with very little modification. Having the ability to "print" one from a 3d printer is just a gimmick. If someone wants to kill someone else, they don't need a gun to do it. Even a toothbrush can kill someone, just ask anyone thats been shanked in prison.
The scary aspect of this, is that skynet can now print guns for its robotic army. 😀