I think LG is right. The issues with bulging are associated with shot size. Larger shot and tighter chokes are more prone to shot pellet "bridging"while entering the choke section. Shot should flow through the choke constriction. However, if a few pieces of shot arrive at the constriction in a bridge formation, it takes a very small extra fraction of a second for enough force to be generated by trailing shot slowing down to collapse the bridge. That small fraction of a second of slow down is all that is necessary to create a gas hammer behind the wad. Thinner and softer barrels are more succeptable to a gas hammer rivle.