Burrard reported....
....He also reported on some cases where the bulge was closer to the breech but not severe enough to cause a burst. In some of these tests two or even three ring bulges were produced. Each moved further down the barrel & were less severe than the first one. This was explained as the result of those Shock Waves Don mentioned as they bounced back to the breech, then forward again to catch up with the still moving charge & produce another bulge. These tests were performed in a barrel tapped for a crusher pressure gauge. In no case was the initial max chamber pressure increased from that of the normal shell being fired....
I think there're a few interesting points, but overall, it's another example of documented bulges without an obstruction, at least as described. Maybe, because of the gas hammer theory, all bulges and bursts are 'ring' bulges to different degrees, and only appear asymmetrical to the degree that the hoop in the area was asymmetrical.
I think there're indications here and there that point to non obstruction bulges. I still know obstructions are a problem, but they may be attributed to more incidences than they might be responsible for. We tend to see examples in larger bore guns. Maybe, a relatively long duration in a relatively large cylinder allow occasional unlucky coincidences of the wave, gas hammer theories and relative localized weak points?
edit to add, I see now that Doc Drew likely added the actual reference pages and references to obstructions.
