I've seen more on headspace lately than in recent years. It seems to go in cycles. Maybe some new writers think they have something to add onto what is really a totally misunderstood item.
Nobody seems to mention that the gauge, bolt and chamber must be SQUEAKY clean before you try to "measure" the headspace.
Nobody even seems to mention that using a headspace gauge, like using a micrometer, requires a certain feel or touch by the person doing the job, and that an extremely light touch is needed.
Let me give you two examples of what I've seen. I certain gunsmith, who was ok at doing basic repair, decided one day to accept the job doing the final reaming on what was going to be a match chamber, to be cut with minimum gauged size. He cut, "gauged", cut, "gauged", cut... until the bolt, in its carrier would just close on the go gauge.
Two problems - while he didn't let the carrier slam home on the gauge, he did let the actions spring gently close on the gauge - way too much force to get an accurate measurement. Second problem is he just blew the chamber out with some shop air between gauging/cutting cycles.
Later when the chamber was closely inspected by somebody who knew what they were doing.... it was found to be about .005 over minimum, or actually a bit past the "no-go" point.
Which brings up - the lunacy of people have with headspace. I mean oh-my-God the headspace is .002 over maxium.... the barrel is ruined, and also... under no curcumstances should be be shot! Total and complete nonsense.
Anyway - be sure to have the chamber, gauge and bolt squeaky clean. Barrel extention squeaky clean. Be sure to turn the bolt onto the gauge as if you were trying to take a reading with a micrometer.... EASY ... its all by feel.
Suggested reading of course, is the chapter in Hatcher's Notebook, as well as the chapter on the Garand rifle's headspace issues in his book on the Garand. It all applies to the AR15's as well.