I still cling to the belief there was an obstruction. The exact location of a burst in relation to said obstruction is also related on how much the obstruction moved upon being hit. If it was solid & unmovable then the burst would have been behind it. In the case of a movable obstruction the burst may actually come forward of where the obstruction initially was, the amount dependant upon the resistance of the obstruction. Another thing to consider also is that at this point in the barrel the inner wall is supported by the other barrel, the point to where they become separated has not been reached. Consequently I do not see it as unusual the bulge was confined to the outer wall.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra