Major Sir Gerald Burrard in the second edition of
The Modern Shotgun (1948), Volume 3, The Gun and The Cartridge, The Diagnosis of a Burst discussed Wave Pressures and the etiology of multiple bulges pp. 364-375.
p. 365
When the explosive charge was placed entirely at one end of the closed vessel (discussing experiments by Paul Marie Eugne Vieille and published in
Etude des Pressions Ondulatoires in 1890) the gases given off naturally rushed forwards along the length of the vessel until the forward layer of gases was suddenly checked by the closed end. When this occurred the gases which were behind the extreme forward layer over-took this layer and began to pile up against it, with the result that the extreme forward layer was compressed with great violence. It was this compression of the extreme forward layer of gases which caused the high pressure
p. 368
Since this wave pressure acts radially outwards the wall of the barrel is submitted to a very severe pressure all round its circumference, and if the pressure is sufficient to stress the barrel beyond the elastic limit of the steel a permanent bulge all round the bore is the result. Such a bulge is knows as a Ring Bulge...
p. 374
If the wave pressure was not sufficient to burst the barrel...the shot charge and obstruction will continue to travel along the bore under the influence of the expanding powder gases, but the wave pressure will be reflected backwards and will occur again at the extreme end of the cartridge-case. It will then rush forward once more (third wave) and may overtake the shot charge...in which case it will act again immediately behind the wads.
It is possible for the third wave pressure to be sufficiently violent to bulge the barrel some way ahead of the site of the first wave pressure, in which case one obstruction would be responsible for two ring bulges.
Radiograph of 2 bulges
I believe questions 1 through 9 should be
"What is the wall thickness at the area of the bulges?"
