The .223 case head is good for long brass life at 75,000 psi.
The .223 is SAAMI registered at 55,000 psi.
That is at least 36% safety margin that can be exploited with handloads.

Contrast that with the 6mmRem or 270Win with case heads good for 67,000 psi with long brass life. They are SAAMI registered at 65,000 psi with 3% safety margin.

What does it all mean?
The loads in a load book for 223 or 223AI are only suggestions for bolt action rifles. The AR15s with non adjustable gas systems may be tuned up for 55kpsi.

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