Yeah, that is a pretty neat idea, and a great deal more practical than his printer ideas, but you don't exactly need a big CNC mill to make a lower receiver from an 80% receiver (80% because 80% of the machining is already done for you when you get it in the mail.) You can finish them out with the sort of 1950's era drill press you might find in any highschool's shop classroom.

If I wanted to make my own lower, and those Ghost Gunners were being sold and shipped today, I'd personally still go the traditional 80% route.

The real part of all of this that makes me unconcerned is that these things are really only interesting for somebody who is intent on staying within the bounds of the law. If you don't care about the law, fucking around with all of this home manufacture stuff doesn't make much sense. Just find a non-felon criminally minded buddy to purchase a complete gun for you (or just do it yourself if you are not a felon). If you live in a country where that isn't possible, will it really be feasible to get the rest of the parts after you've milled your lower? Does the UK allow you to purchase all non-lower receiver parts of an AR-15? Would they allow you to receive the specialized mill in the mail in the first place? I honestly don't know.