[verum ago]

Did you read the rest of the article? they theorize basically an extension of what you were getting at, that the failure was actually caused by an obstruction of the barrel (which is what the photos make it look like):

"There are a ton of folks out there who constantly load and unload the top round in their magazine, and then complain that the overall length is changing. In fact, such battery of the case neck can weaken it so much, that the shrinking tendency will rebound, and all of a sudden, the bullet is gone!

Where? Down the bore! The darn things pop out of the case upon chambering and fly down the bore to create an obstruction for the next round."

[headstamp]

They claim there was no obstruction but it looks like there was as the barrel barber-poled ahead of the chamber.

I would think a primer would blow out before this would happen IF it was a deep seated bullet instead.

This looks like a classic bore obstruction with the attending air compression by the next bullet splitting the barrel.

If the chamber wasn't jugged, I'm calling this a bore obstruction like you are surmising.

[ExpatGator]

If this is the case, and the the malf occured on the 1st round with the bullet dislodged from the stretched out casemouth, then the pressures would have been lower and no catasrophic failure would have occured.

I accept that I could be wrong on this, but it just does not make sense as put forth.