Remember also that if the two loads produce identical ballistics (same weight of ejecta @ same velocity) at different pressure levels the one with lower pressure will hold that pressure just a bit longer.
Time is the reason a lead crusher does not give a true PSI reading, always lower than recorded by a Piezo Crystal. The duration of the peak pressure in a gun chamber is so short it is nowhere close to that amount of static pressure.
I am in the camp that says pressure is the factor for determining barrel wall thickness to prevent a burst barrel. Ejecta weight (Shot, Wads & Powder charge) & Velocity determines the stress put on the action & also splits stocks.
