One thought.  If the ammunition were allowed to bake in the sun during a sustained outing and the gun had been fired a lot so it was extremely hot. Then one of these rounds were chambered and allowed to cook for bit before being fired.

This could cause a substantial spike in pressure.  Combine this with the fact that the round may have had a slightly (but in specs) heavier charge, 5.56 in a .223 chamber, a dirty (from sustained shooting not carelessness) rifle including some carbon build up in the chamber and on the bolt face and you have the potential for failure due to excessive pressure.  Any one or maybe two of these items would not in itself cause a failure but line them all up in one equation and you have the potential for this type of failure.

Just a thought!  

DonT

Whew, I have a hard time getting through to this site. Anyways, thanks for the concern.

With a kb this bad, the shooter was very lucky to only have minor scratches. Basing from your replies and our observation and unscientific analysis, we think that bullet setback is the most probable cause of this overpressure. and in case if you guys are wondering, we can have our ARs here in any length we like and with the happy switch too. :) we just have to get a license for it.