I know there are a lot of folks who are checking their fired brass with an RCBS or Sinclair device, and think they are measuring rifle headspace. But it is just as likely they are not.
We had one gentleman here who is sizing his 30-06 cases .008" long, because that was is the length he measured with his RCBS tool after they came out of his M1 Garand. The sized cases were so long, he could not use them in his M1903's or bolt guns.
I highly suspect that his long cases are being swaged into the chamber when the M1 bolt closes.
(And as he turned out to be such a very nice guy, it troubles me greatly to think he might have an out of battery slamfire)
Best way to measure rifle headspace is with a rifle chamber headspace gage.
Incidentally, my Wilson cartridge headspace gages, you drop a rifle chamber gage in one of those, the "go" gage lines up with the bottom mark, and the "no go" lines up with the top mark.
As mentioned earlier, changing to different brass, reloaded different amount of times, the springback will be different.
I excavate my cartridge headspace gages each time I am sizing service rifle brass. Especially if it is once fired range pickup brass, and I check to make sure I am sizing to gage minimum. Be surprized how often I have to adjust the die.