Found this in a previous discussion:
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
Note that if the surface of the powder is not perpendicular to the run to the bullet then the shaped charge effect is broken up by the random nature of the pressure "walls" formed off the uneven surface. Also note that Dell was able to ring a barrel with no wads at all involved merely by shooting straight up thus starting with a level surface on the powder.
