"I guess that 3D printing can make dangerous items like cars, motorcycles, and adult novelty products they can make weapons too."
They can, but if I wanted to make a firearm from scratch I'd use a CNC mill, not a 3-D printer. Same basic idea (point and click construction) but you end up with solid steel or aluminum parts - not plastic.
If I wanted to make an unregistered firearm I'd start with an 80% frame and use a specialized jig along with a hand drill or preferably a drill press and then order a "parts kit" to flesh out whatever firearm it is I'm building.