....It seems apparent that there was also some prior leakage of power gasses due to this combination of defects, and that was causing a separation of the rib braze joint. When enough surface area of the braze joint was compromised, it finally let loose....
I think it's reasonable to guess that the defect progressed, fresh broken steel or brazing would be bright.
I wouldn't think this is propellant gase damage or coloration. The location would seem to indicate that a hull would seal against the chamber wall. This pit, or defect, or inclusion would have had to have been past the 2 3/4 in. hull length. Also, I'd think if gases came back along the rib extension stub like the darkness suggests, the shooter might see fouling on the breech face and rib extension.
