Your issue is common with those who have a match barrel with a match chamber installed on their AR15 and use brass fired in a different rifle.
The cure: a Wilson type cartridge headspace gage. You cannot measure the base to shoulder of brass fired out of a gas gun and have a true chamber headspace measurement. This is because gas guns extract when there is still residual pressure in the chamber, and that stretches the cartridge case. But, you can measure sized cases with this cartridge headspace gage. And I recommend you size to gage minimum for a gas gun.
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I also recommend you size your gas gun brass with a small base die. It makes a difference.