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Is it the same bolt with two different barrels? If so, I would think that the bolt face has been cut to deep, or the lugs set back too far.

Things that would increase pressure - projectile seated to deep, short lead, tight throat, excessively low case capacity. Shoot your loads through another AR and have a look. I have only ever seen...

That load sounds piss weak honestly. With a proper 5.56 chamber, you should be able to run 26gr of 2206h under a 55gr bullet no problem. In all of the AR baŕrels I have had or have, anything less...