You don't 'set headspace' on a home built AR. The headspace is determined by the barrel extension which is basically permanently attached to the chamber end of the barrel. Usually, the barrel maker will chamber using a 'bench bolt' which is supposedly of proper dimension. The bolt maker cuts the locking lugs to a specific dimension(within a specific range).
OK, there WILL be some dimensional variation even within 'mil-spec', that's what 'tolerances' are for. Given a unique set of max/min tolerances, there is a possibility of encountering a 'no-go or excess' in bolt face/chamber dimensions. I see this occasionally and depending on use/expectation of the AR being assembled, there may be a need for some parts swapping to reach that goal.
GENERALLY speaking, the potential for such an issue is very small. A couple times I've found a combination of bolt/barrel that refused to consistently chamber a variety of factory loads-not acceptable. I've also found a few combinations that would chamber and fire reloaded ammo that nothing else would but didn't cause problems such as backed out primers or case head separations--good enough for government work.
I've also heard stories about a squad cleaning procedure that entailed dumping the entire squad's BCG's into a bucket of diesel fuel to soak and then just grabbing any of those to go back into random M-4's. I wouldn't but Uncle Sam seems to be OK with this.
I don't own a set of .223/5.56 headspace gauges but have successfully assembled 'several' AR's by common sense 'feel' w/o any issues. As said, this may involve some swapping. If you don't have multiple combinations to swap around that's a different scenario. That said, once a bolt/barrel set is fired more than just a few times, it IS a 'set' and stays together-no bolt (BCG) swapping between multiple uppers. Some users are OK with one BCG and multiple uppers but not me.
