I'm no "expert" or metallurgist either but it sure looks like the barrel was already in the process of letting go based on what looks like oxidization that had formed along pre-existing fractures. A lab with a high power microscope should be able to identify the tell-tale "beach marks" of fatigue if this failure was in process over time rather than an abrupt event. I must add that I have never examined such in Damascus steel. If the staining is indeed oxidization I would prefer hearing that such came by way of fatique fractures and not that which could be associated with pitting. The latter could make any fan of Damascus barrels a tad nervous.