Miller, you answered your own questions and concerns when you brought up the lead crushers and piezoelectric transducers.
Typically, these are installed into a test barrel in a spot on the chamber where pressures will be greatest during the firing cycle. We all know that the lead crusher values give a lower reading, and that there is a percentage added to correct for that in order to equate it to the pressure the same load would generate if you were using the piezoelectric transducer. The example of primer flattening or flowing under extreme pressure is simply an exaggerated way of illustrating that there is a definite rearward force exerted by pressure. It doesn't matter if one load is 8000 psi and the other load is only 50 psi less at 7500. The higher pressure load will always have the higher rearward force, unless we can suspend Pascal's Law.
It also matters not that the pressure is not static, or that it is sustained for a very small fraction of a second more or less. The same correction factor for lead crusher testing is generally used across the range of cartridge pressures and with all different loads and powders. We all know that the lead crusher pellet or the piezoelectric transducer is going to tell us which load generates the higher pressure. And if that measurement were to be taken at the rear end of the cartridge rather than an inch or so from the breech on the chamber wall, then it will prove to us that the higher pressure cartridge stresses the action more. And as a machinist, you are certainly aware that engineers commonly add material to provide more strength to mechanisms which will undergo sudden shock loading. This is further recognition that the higher pressure load stresses an action more, all else being equal. And it takes us back to the very real fact that purpose built double rifle actions are built with more material in critical places than their shotgun counterparts. You won't find many gunsmiths doing double rifle conversions using Ithaca Flues or Tobin actions.
