Jones;
You are correct, total impulse is a more descriptive term. Was mainly just trying to make the point that Max Pressure is not the final outcome on this Total Impulse. You can with many loads have more total impulse while maintaining a lower Max pressure.
While I do not actually have the necessities of checking it I do believe that in most cases "IF" using the same identical charge if the max is lowered then the total will be lowered as well. Thus when a weak crimp lowers the initial pressure rise, lowering the max it is also going to result in a lower total impulse thus lowering the velocity as well.
I am thus in total agreement of your assessment of new versus older shells & particularly where slow powders in cold weather come into play..
