Burrard also discusses the bulge in an obstruction of this nature occurring forward of the location of the obstruction. Briefly his explanation was that when the load hit the obstruction it moved it down the barrel but velocity was checked enough the expanding powder gases behind the wads "Piled Up" to produce a local high pressure area. The barrel could be bulged or burst depending upon the severity of the Check in velocity.
He also mentioned that tests were done with both solid & hollow obstructions & the severity of the bulge or burst was totally dependant upon the weight of the obstruction, thus eliminating the theory that obstruction bursts were caused by Compressed Air between the load & the obstruction.

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