Why would I ever want to make a plastic gun, when I can go down to any urban liquor store and have a real, metal one stolen-to-order for me for a couple hundred bucks. Just ask the guys lounging around in the parking lot.
Just as anonymous and the gun will work a lot better...
They can 3d print the parts for a metal gun. They have 3d printed the parts and assembled a functioning 1911 gun. The only thing keeping someone from being a personal gun factory is the cost of a 3d printer, the materials for the printer to make the metal parts, a knowledge of all the parts, and knowing how to use one.