Michael Guslick, an amateur gunsmith who has written extensively online about the considerable challenges of 3-D printed guns, said people had been experimenting with homemade guns for some time. He said the most notable example was the zip gun, which is made from off-the-shelf plumbing parts. (Not surprisingly, the schematics and instructions can be downloaded online.)
"This is just applying a different technology to something that is already being done," he said. "But making one on a 3-D printer is a lot of work when your local plumbing department is so close by."
So the bottom line is making a barrel or reciever with any accuracy still requires an experienced machinist....and wait, experienced machinists have ALWAYS been able to make guns.
I have only rudimentary machinist skills. They are not difficult to develop. I have no doubt that I can make functional guns with a little effort.
I have made multishot guns from plumbing supplies (legally) and they functioned just fine. The highest technology that I used was an electric hand drill. You do not need rifling or precision for functionality. Crimes are almost always committed at distances measured in feet, not tens of yards.
Yes, if you want to duplicate a .50 cal machine gun, an experienced machinist would be advisable. To make functional guns suitable for crime, they are not necessary.