I was told that armorers correct headspace by replacing the bolt, but gunsmiths who works on ARs generally can correct headspace using a lathe and the correct cutter.


NEXT QUESTIONS.
ARs are available in 5.56mm and .223 Remington chambering.  The chamber dimensions are different between the two.  Are the headspace gauges interchangable between 5.56 & .223?  I know there are .223 field, go, and no-go gauges.  I also know there at one time was a military surplus headspace gauge for 5.56 for sale in the Equipment Exchange.  Would the military surplus gauge actually be a no-go or a field gauge, or is it entirely different?

Has anyone ever had experience doing so, or had the headspace of an AR corrected by a gunsmith using a lathe & cutter?

about your headspace gauge question:
they should be interchangeable.  they measure from the shoulder.
the difference in dimensions between the two calibers is in the throat.
IIRC.

If your headspace is excessive, I can't think of any surface you could remove material from to tighten it.  The AR-15 was designed to rely on properly dimensioned parts for headspacing, and I don't think there is anything you can do about it except the officially approved step of replacing parts.

Headspace gauges are not technically the same between .223 and 5.56, but they are close enough that many people use them.  Military gauges are available in go, no-go and field, but many people state you only need field.  True 5.56 gauges are a bit tough to find; I am told the ones stocked by Brownells that claim to be 5.56 are actually commercial .223.

Actually, I have reconsidered.  You might be able to take some material off the rear barrel face and screw the barrel deeper into the extension to tighten the headspace, but that would offset your front sight and gas port to the side, creating a visible cant to your entire front end.

The only way to tighten it up is a slightly larger bolt won't do it is to remove the extension, remove material from where the extension seats against and then reinstall the extension. That opens a whole can of worms as the gas port is no longer lined up with the indexing pin on the extension. You can plug the port and drill an new one but if the front sight is pinned then then you have to move that.
Not worth the time or money to try and fix a worn out AR barrel.

I remember a few years back of a bunch of  barrels being sold dirt cheap. Why? The extension was missing and unless you have the tooling and time it was not worth it to try and fix them.