Could someone please explain how headspace is adjusted in an AR. It was my understanding that it had to do with the fitting of the barrel to the upper, and the bolt doesn't have anything to do with adjusting it.  I know this is not true of most other rifles.

I am wanting to know if I could switch a bolt and carrier from 1 rifle to another?  Or buy an upper from one company, and the carrier group from another, and not have a problem with headspace.

Thanks....

WOW, I was going to post the EXACT same question to this group myself - but you did it for me- 5 minutes before!

I was thinking of buying an upper from 1 company, and a complete bolt assembly from another, and didn't know if the issue of having to headspace would come up. Don't know if it's mandatory, a good idea, or not needed at all...

I *think* that the headspacing all occurs between the upper and barrel, and that bolts are all machined to mil spec or better and are identical and should work ideally. Again, that is what I think, but that is why I ask here to find out for sure.

I would be deterred at the moment from going that route if I had to engage in headspacing or the purchasing of the go, no-go, and field gauges.

Headspace is determined by a point on the shoulder in the chamer to the breech face on the bolt,,,,,,, thus locking lug thickness on both the bolt and barrel extension along with the bolt' face all play together.  

Bolts and extensions are held to a tight tolerance, so only thing left is depth of the actual chamber.

An upper only holds the barrel on the AR15/M16 family.

The headspacing on all new parts (bolt, barrel extension), whether or not from the same manufacturer, *should* be ok, unless something is grossly out of spec.  Once the parts get worn/broken in, the gereral rule is not to swap used bolts with another rifle.  

So, if you follow that rule, your new barreled upper from manufacturer A should work fine with a new bolt from manufacturer B.  By all means check it for your own safety and assurance, though.  

This topic was covered a few months ago in here & one of the experts said that headspacing is not an issue.  Don't take my word for it because I've never done it & I'm not an expert.  If Search ever comes back do a search for it.

There is NOTHING you can do in assembly or misassembly, installing the barrel onto the upper receiver that has the slightest affect on headspace.  That is determined by only two parts, the barrel (and its extension) and the bolt.

YOU cannot adjust the headspace on an AR except for one thing... you could ream the chamber and increase the headpace.  Or you may remove material from the back of the locking lugs on the bolt.  There is no way to reduce headspace if it is excessive.

But really, if all parts are new milspec, even with "stacked tolerances", headspace will not be a problem.

Edited to add:  The bolt carrier has nothing to do with headspaced.

I have switched bolt carrier assemblies (which include the bolt) among AR's.  I have purchased all parts from different manufactuers to build the exact rifle I wanted.  I have switched around bolts and bolt carrier assemblies in order to troubleshoot other peoples' AR's.  

With all of this I have never had a "headspace problem".