That let go at the breech.
Look at the shell.
Look how thin the chamber wall is at the rib.....one of the most important places we cannot measure wall thickness.
Look how compromised the chamber to rib solder is.....it is black and crumbly, looking just like the little chunk missing about half way down the chamber I bet that was there a while...little chunk or pit let go, eat solder from inside out and after some time.....boom.
I see no ring bulge, I see the back of the chamber letting go and peeling the barrel forward.....not saying for sure it was not obstructed but that was going to go....sooner not later.
I am in agreement with 775. There was a pre-existing crack/break, where the barrel was milled for the barrel extension, at midchamber. That point is clearly shown in the picture as having previous corrosion while the rest of the fracture surface is fresh metal. This, in my opinion, is the root cause. Whether there was an obstruction to speed it along is to be determined. That missing chunk of chamber would be nice to evaluate.
John
