Considering the appearance of this bulge I would say its almost certainly an obstruction bulge. A thicker wall would have offered more resistance to bulging of course, but the thinness was not the root cause of the bulge. .077" is not really that thin at 4" from the breech as pressure has fallen considerably by that point. Where is the 2nd bulge? It has been proven conclusively that one shot with one obstruction can produce more than one bulge in the event the barrel is not burst at the first one.
