...."IF" you fail to take into consideration that both Force & Time are involved as a static pressure level is not reached you simply have not looked at the entire equation....
Only conversation Miller. In this example, I think time is a constant. From a standing start, both high and low pressure shells have identical muzzle velocities, so they must have spent the same amount of time in the barrel.
Both payloads are still accelerating at muzzle, because I believe PSI's have been shown at the muzzle. But, to keep the force, the area under plotted PSI curve, equal, the high pressure load would be accelerating at some lesser rate than the low pressure loads towards the end of its travel down the barrel. To keep the total force on the payload equivalent, the higher PSI's likely created a higher rate of acceleration right at that point it was measured, but the rate trailed off quicker than the lower PSI load.
As might also be the case in this example, the total force is also a constant, so the thought is, does applying equal forces in different ways, matter? I believe your point is related to the shorter duration of the exposure to the higher PSI, but the gun was subjected to the higher PSI none the less.
