Ok now.....Lets see if this will fly.
Support of the bolt opening under pressure is one that I was reluctant to consider. Then I read other kbooms and the results are similar.
There are now two ways the bolt can be opened in my opinion. One is for normal operation where the gas opens the bolt. We all understand this.
Now, it is clear the case blew first due to an obstruction of some type, now the bolt unlocks and blows the mag etc.
If the pressure is great enough to blow the mag and bulge the lower, then that gas venting directly inline with the face of the carrier was sufficient to move and unlock the bolt. The extractor couldn't move the case under 50,000 psi.
Do any of the members have contact with any of the ar 15 Manf. that can evaluate this possibility.
If this is what happened then it could happen to any of us. Jim