....The obstruction must be of a size and/or mass to create a gas hammer which then creates a bulge....
An obstruction is certainly a problem. Not long ago, I believe Toby B. commented on these forums about the various proof failures he had experienced over the years. Bulged barrels were mentioned specifically. Of any incident that created a bulge in a shotgun barrel, it's probably safe to conclude that an obstruction would be documented to be not presents, and the specification accuracy of the ammo should be beyond question.
If it can happen under the controlled conditions of proof testing, maybe over pressure alone can cause a shotgun barrel bulge out in the general shooting community.
