Razor,
First thing to be clear on, is that H/S is a two part process; gun AND ammunition. We have two different definitions here that need to be borne in mind when discussing this topic. First, the ammunition has SAAMI or CIP specs on what is "correct"; a range of around .006"-.010" on where the datum line is in relation to the case head. Secondly, we have the chamber, and the obverse measurement; the face of the bolt to the datum line in the shoulder area of the chamber (or whatever other means are used to stop forward movement of the cartridge in that chamber). These two elements have to work together to determine what a handloader is referring to when we talk about "correct headspace." In otherwords, a chamber that shows grossly excessive headspace when checked against a chamber gage, is no problem at all if it's fed ammunition which is matched to it.
As far as the ARs go, yes, they do have headspace issues that have to be contended with, but that's usually on the barrel maker, and how he fits up the barrel extension. Once on, it's "fixed" but you still have to make sure that your ammunition matches what's in the chamber of that particular rifle. This is why we're always talking about the gages, checking shoulder set back, etc.. It's because these are variables, and ones that you have some measure of control over at the loading bench. Yes, use the gages, verify what you've got coming out of your rifle upon firing, and then use gages to set up your dies to ensure that you're giving them enough room to function, without pushing the shoulder so far back (again, we're only talking a matter of a few thou here) that you're killing your brass. Everything's got to work together, and you're the controlling factor on making sure that happens correctly.