The conspicuous bulge near the breech-most end of the split sure looks like an obstruction to me. For sure, significant yielding took place before ultimate rupture.
The highest pressure at 16" from breech (eyeball estimate assuming 30" bbls), in my modest collection of pressure curves, is just over 1500 psi. Even if the walls are down to 0.020" (and they look thicker) we're talking ~ 28,000 psi hoop stress there. That's more than I'd want in old bbl steel but not likely to cause yielding, IMHO. Cyclic fatigue, possibly, but the big bulge indicates to me it happened in very few cycles (unlikely) or just one, probably.
