I fl size brass and use that brass to set headspace when changing barrels. In 223, 22-250 and 308 the fl dies size the case to ~ .002 under the RCBS Precision Mic zero.
On occasion I find a case that has correct headspace per Precision Mic, but when fl sized, still won't go in the gun.
I take a spare barrel, put the case in the chamber, and tap the head with a plastic mallet. Tap, slightly!
Each tap, try the case in the gun.
All cases eventually go in all guns.
I don't know WHY cases that measure for headspace correctly don't go in some guns, but it happens to me occasionally.