I've considered all this.
The rings form where the base of the bullet is in the cartridge, so it would appear that rifling engagement has not yet occurred. In fact, there has been no bullet motion.
Inertia is inertia. An ounce is 437 grains, no matter if it's shot or ball.
True, shotguns do operate at lower pressures than rifles. These are black powder rifles though, and the difference isn't that huge. A trapdoor 45-70 load is in the area of 17K PSI for instance and a 3 1/2" 12 gauge is SAAMI specified at 14K PSI. They don't seem to ring chambers like rifles.
Kirk at Shiloh has stated that if you use the standard load of AA 5744 in a 45-70 case (which fills the case about 50%) and then top it with a wad in the mistaken belief that this is necessary to keep the powder in contact with the primer it will ring the chamber EVERY TIME. That has to be caused by a mammoth air pressure spike between the wad and the bullet and not the natural chamber pressure of the expanding powder gas. The load works fine with no wad. People use it regularly.
There's plenty of air in a shotgun shell between the over powder section of a plastic wad and the shot cup. There's a collapsing section between the two, but it sure does not seem substantial especially in the case of something like a Windjammer. 11,500 PSI in a standard load would seem enough to just smash that flat as if it wasn't even there.
What happens to the air? It can't go anywhere, just like the air trapped a in rifle cartridge that appears to compress and expand some really strong steel. I can't imagine what the actual pressure might be to actually do that. It must be enormous.
Obviously, this does not happen in a shotgun and shotgun chambers are generally not as thick and strong as rifle chambers.
I'm certain you're right about things happening more gradually in a shotgun, but this just boggles what little mind I have left.
