Bill,
I think there are some extreme examples of pressure differences with same weight. The whole damascus reloading premise is based on low pressures and the flagship argument is that you can't pick a modern over the counter load that is acceptable based on velocity. So, some guys are claiming reloading can give them sub 6ksi pressures with velocities/payloads equivalent to modern loads constantly spouted/touted as max SAAMI pressures. If true, that's pretty real life.

Frankly, I don't believe for a minute that every factory load is max SAAMI, some not even close. I've measured a few and those were down a couple thousand psi. But that's still a substantial pressure difference for same/similar velocities/payloads.