I'd love if you ever have the time to explore the concept of buying a hobby-level machine capable of turning out a functional 1911 or AR - even if it taxes the CNC and takes a week to mill the frame. When Miller started III Arms he entered with a Tormach - an "Entry level" machine in the world of real CNC - but I'd love to know if you think there is an answer out there for the Patriot who wants the ability to load some pre-constructed code, add steel blocks, and end up with a defensive tool.The answer is that they probably are all capable. It's not falling off a log easy, and some operations might be a bit scary, but I think there are tons of people using CNC-modified Grizzly and Harbor Freight mills and lathes to do this. I would love to have a nice Tormach machine, but they are serious money for a hobbyist. As a runner up, the Chinese company Sieg produces the majority of the cheaper imported machine tools, with differing capabilities, sizes and costs. I've mentioned this one before and with the exception of not having the room in my garage, I probably would have bought one of these by now. Seig makes this one, as well as the machines you see sold by various tool companies like Grizzly, Harbor Freight, and others. My machines are currently all Sherline, with options by A2Z CNC. Both the Sherline parts and the A2Z CNC add-ons are made in the US. While the Sherline's standard work envelope is small, they are more accurate out of the box than pretty much anything else you can buy until you hit that Tormach, to the best of my knowledge. A2Z has made their own versions of the Sherline machines which are capable of machining bigger things. See their website.
As you say, with a smaller or less rigid machine, you can make more shallow cuts than on a bigger machine, and you take longer. CNC is ideal for this; it never gets tired, never miscounts turns on the hand wheels.
Of course, I've got a permanent post on how I built an AR-15 from an 80% lower (my rifle and the machine I made it on, below). I haven't mentioned it, but thanks to The Redneck Engineer, I have some "0%" lowers (forgings) that I intend to machine all the way. With the personal issues that have complicated life this year, I haven't been able to get to it, but I always thought of it as a "round tuit" project. Judging by "The Book", I think this task is possible with my Sherline tools, but would be easier with a bigger capacity lathe. This guy has a pretty thorough look at the process - on big machines.
Of course, the AR platform, while a fine rifle, isn't the be all and end all of guns. You might want a 1911. There are 80% 1911 forgings to complete on your mill. KT Ordnance of Dillon, Montana, is a prominent source. This gets around the potentially nasty problem of cutting out the magazine well, which is a long reach for an unsupported cutting tool. KT says the BATFE has ruled what constitutes an 80% lower, and you'll need some special cutters, which they'll sell you. This guy has a decent write up. KT sells an 80% Ruger Mark 1, .22LR pistol!
You will need information and places to hang out and just read. Of course, AR15.com is a go to site for all things AR. But don't miss The Weapons Guild or CNC Guns. There are many forums devoted to particular machine tools where you can learn the ins and outs of your particular machines Yahoo! Groups has several. YouTube has many videos of small milling machines converted to CNC, and there are commercial options, too. The advantage of going with one that was sold as a complete system, like the Little Machine Shop tools mentioned above, is you have one seller to go to for support. When you put it all together yourself, you can have problems isolating problems and getting it all running; with one of these, you may need to clean it up to remove protective grease, but it should work when you power it up.
Hope that answers it. If not, let me know!