Obviously those barrels which Bulged in proof were not likely to have been obstructed. I "Believe" that normally proofing is none with a powder designed to stress the forward portion of the barrel to a greater extent than would be done with normal shotgun powders. My take would be the proof load exceeded the elastic limit of the steel at the point it bulged.
I would also expect that fully 98+ % of barrels bulged in use, either hunting or clays were caused by an obstruction. Also keep in mind that when an obstruction is hit the pressure in entire bore area behind the obstruction is not raised to the same level as At the Obstruction. You get a localized pressure spike near the obstruction, depending upon the severity of the obstruction. In some cases the bulge will actually occur a bit forward of the actual obstruction, showing the obstruction was moving before the pressure spike occurred, the gases simply caught up with it.
In the case of the Lefever I mentioned I feel the "Dent" was severe enough to cause an obstruction. I cannot think of any other reason the side opposite the crack would have been raised. Personally "IF" a barrel of mine gets a dent, I want it raised. I'd rather be safe than sorry.
