Miller, Pascal's Law is a fact I can provide to prove that there will be more force or stress exerted upon the action of a gun firing two loads of equal velocity and equal mass of ejecta. And THAT was the question in the original post:
Does pressure by itself stress an action or is stress more related to the velocity of ejecta? In other words, if two loads have the same ejecta velocity but one develops 2,000 psi higher pressure do the two have the same stress on an action?
Pascal's Law states that when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container. The container is the shell or cartridge, of course, and the fluid is the gasses formed from the rapid burning of the powder. The pressure does not just act perpendicular to the axis of the bore. The same pressure that requires thicker barrel walls at the chamber is also pushing equally on the area at the inside of the rear end of the shell, and pushing with equal force upon the end of the wad containing the shot. The same force that pushes the wad and shot charge down the barrel also pushes equally in every direction within the shell. And to briefly return to the example of primer flattening in a rifle cartridge, this would still occur with excess pressure even if there was zero headspace. In fact, with really hot loads, we see this rearward pressure actually causing primer metal to flow back into the firing pin hole.
The difference in stress or force upon the shotgun action from the higher pressure load may not be all that great, but it is undeniably greater in the higher pressure load. And that was the original question.
