I understand the problem space, I've built my own gun parts using both 3d printers and traditional machine tools.
The thing is, criminals aren't hobbyists. They're not going to goof around at the range for 8 hours trying to tune in a mechanism. As long as traditionally produced guns are the price of a 3D printer, (or a busted pickup truck window away), there's really no reason for your local gang member to do an arts and crafts session.