Sticking to the original question, all that really matters is that if either your lead crusher or piezoelectric transducer was placed at the breech end of the shell, the higher pressure load would still crush the lead pellet more. And the piezoelectric transducer would still show more force transmitted from the higher pressure load.
We can play with all kinds of what-if's and hair splitting over milliseconds of peak pressures But the simple question was about which load of equal velocity and equal mass of ejecta would stress the action more. Going back to rifles, this is why a Winchester High Wall is suitable for higher pressure cartridges than a Low Wall. And it is why firearms designers have chosen good steel to make shotgun actions versus many other materials that would be cheaper and easier to machine. It is also why bolsters are added to reinforce the frame of double rifles.
