With the machine tools in my garage, I can build an AR-15 in an afternoon. But I don't have particularly good tools, it's easier than that and requires no skill with some of the other options out there:
You can, for not much of an investment. get a computer-controlled milling machine that will produce the relevant bits for AR-15s without skill needed:
https://ghostgunner.net/
But that's if you want a traditional metal firearm. You can easily 3D print quite a few firearms, and again, without much investment or skill:
http://www.printedfirearm.com/category/receiverslowers/
Interestingly, some firearms receivers can also simply be carved out of wood, or glued together from laminated cutouts.
And that's if you want a nice AR-15. If you want to build an AK-47 at home, you can basically make one with a hammer and some rocks.
For some sound reasons, the AR-15 platform is the top-selling rifle in America, and has been for some years. There are ~10 million of them out there. It's not going to be easy to put that horse back in the barn.