Rocketman, from the rivelling patterns I have seen, this bridge or alignment of pellets which exceeded bore diameter, and then either broke down or deformed the steel barrels, would have to be circular and slightly convex in nature rather than a linear string of shot. Also, it would have to form almost perfectly perpendicular to the bore as any other angle would easily break down before it could deform the barrel walls. Then, it would have to reform very quickly after bridging and creating the first ring bulge, and go on to do that several more times in very rapid succession. It's also hard to imagine relatively soft lead spheres being able to bridge and displace that much steel in a repeating ring pattern without deforming and collapsing past the point of exceeding bore diameter itself first. And if this was the cause, wouldn't the rings, especially the first couple, show circumferential dimples caused by the individual pellets at the perimeter of this circular obstruction where they touched the barrel walls? Another thought... if gas hammering or multiple detonations of unburned powder or some other gas pressure was responsible, wouldn't Pascals law dictate that the pressure would act equally on all internal barrel surfaces rather than short, repeating sections?
This is not intended to be a critique of your theory, but rather some food for thought to consider if you go forth to tackle the problem. Has anyone documented rivelling with modern shot cup wads, or did/does it mostly occur with older style under shot wads where the shot is not isolated from the barrels by the plastic wad fingers? The hydraulic deformation theory caused by excess oil in the bores still appeals to me because we know how a fluid is relatively incompressible and could easily bulge or burst (or ring?)tubing if it wasn't displaced, but instead got trapped under or at the leading edge of the wad. I suppose we could throw in the weird things that could happen under high pressures and velocities or maybe resonant frequencies and damaging harmonics. I once saw some distinctive rivelling type patterns in a pile of thin wall steel tubing which had failed hydro-testing at approx. 3000 psi.
I'm just throwing some ideas at the wall to see what sticks. Hopefully some additional food for thought will help you find some definitive answers that will benefit all of us.
