5.56 barrels

[Madusa]

I’ve had ammo fit case length gauges but not chamber. Girth can be an issue also.

[BPTactical]

I have found my best headspace gauge is a factory loaded match round from a REPUTABLE manufacturer such as Hornady, Black Hills or Barnes.
They are manufactured to very tight specifications.
Bolt should close on a round, take same round and put a piece of vinyl electrical tape on the case head. Bolt should either not close or close with effort and smudge the tape.
If the bolt closes then try it again with a second layer of tape, it should absolutely NOT come anywhere close to closing.
I like Scotch 88 tape, it is about .003 thick.