"but aren't the parts of a gun which can be machine printed, also fairly easily manufactured using widely available metal working tools?" No, it's quite differ

"but aren't the parts of a gun which can be machine printed, also fairly easily manufactured using widely available metal working tools?" No, it's quite different. A metal work workshop is not something the typical urban criminal has access to. Yes, I realize there are many people in rural areas who have such workshops, and shared workshops in urban settings, so theoretically yadda yadda yadda, but the practical (socio-economic) reality is that those who want firearms for criminal purposes in the vast majority of cases do not have access to these tools, and/or lack the knowledge to reverse engineer and adapt available plans for metal weapons, and manufacture them. It's quite hard to make a metal gun using a traditional lathe and milling machine, and even when using a high-end CNC. 3d printing on the other hand requires no more than the average high school kid's computer knowledge, plus a few files downloaded from the internet. The barrier to entry is orders of magnitude lower. The costs are vastly lower, too - you can get started printing a gun (although I don't think guns made on MakerBots today will actually work) for ~ 1000$. That is not much more expensive than a weapon on the black markets costs (to those who have access to such a market). You'd spend an order of magnitude more on a metal workshop, and that's not even counting the materials and auxiliary tools.