1. Wikipedia has a decent discussion of head spacing:
It is critical that you understand this for many things aside from re barreling, such as reloading.
2. There are plenty of youtube videos that explain how to re barrel an AR. It does not require a lathe, just the correct wrench and means to hold the upper receiver. It is strongly suggested that you check head spacing after doing an ar barrel install, but if it is incorrect you simply send the barrel back and complain that it was not mil spec (assuming you did everything right). This assumes you are using a pre-chambered and pre-threaded barrel, if you start from a barrel blank then you need a lathe. Starting from a blank is only done for high-end match rifles, plenty of folks buy a pre-threaded, pre-chambered AR barrel and do the job in their garage.
3. There are also short-chambered pre-threaded barrels sold by a number of barrel makers, like pac-nor. These will supposedly fit standard threads on the actions they are intended for, and then you finish ream by hand in many instances without a lathe. I know this has been done by a lot of people on their own with old mauser actions, but I don't know about other actions. This way, you just need a barrel vice, action wrench, reamer and reamer handle, and headspace gauges.
4. Savage rifles use a nut system whereby you also don't need a lathe. Pac-nor sells a nut system you can use for Remington rifles as well. I have never used these, but my understanding is the nut replaces the receiver shoulder as the crush point for the barrel shoulder, and you simply adjust the nut until headspace is correct on a pre-threaded and pre-chambered barrel. Tons of folks do this with their savages, and again I think there are how-to's on youtube.
Otherwise, the rifleman's journal blog just did a great 4 or 5 part series on how r700 clones are re barreled, starting from a barrel blank:
There are also youtube videos all over showing not only how to re barrel from a blank on a r700, but also how to true up the action.
there is a blogging gunsmithing student, I think the blog is called nerdgun or something like that.