You have to be an experienced metalworker to make one that way. A computer doesn't do it for you (at least not yet). Whereas any idiot can download a file and send it to a 3D printer.
Well not so much. If you've got blue prints or a replica of any metal part you can go to any local machine shop have them make it for you. Of course you are going to be paying $150+ an hour for labor, but it can be done. I had to get a lot of parts machined from raw materials on an old motorcycle I restored once. There were not after market parts for it. I had to get everything that was broken or worn out made at a local machine shop. Nothing stopping me from handing them blue prints for an ak-47 receiver. Machining a gun barrel might be a little more obvious, but barrels can be bought on ebay and other online shops. Usually only receivers and frames are what is regulated, but frames can be made custom out of wood or plastic.
Usually these 'homemade' guns buy a lot of professionally made gun parts such as the rifle barrel and just make their own lower receiver, which isn't exposed to the explosive high pressure part of a gun.
Disclaimer: I’m familiar with this only because I have to check my firearms when I’m flying.
Everyone there had to wear a white shirt and pants. There was only one entrance and there was a metal detector right in front of it. But what all the "cool" kids were doing was seeing how big of a knife they could sneak through it. And there was a couple of 3 inchers that got through. It mattered were the knife was and how you held your hands as you went through, but can't remember the details (this was back in 2001).