Ok, I'm going to throw my $.02 in....

I haven't seen anything mentioned about powder burn rate. Assuming equivalent payloads and velocities, I would expect that a faster buring powder would create higher peak pressures. My rationale is this: Pressure is created by expanding gases. If the area the gas expands into is larger the pressure will be less. With a slower buring powder the "ejecta" starts moving down the barrel as pressure increases thus increasing the expansion area and resulting in a lower peak pressure. I would also expect the "felt" recoil with a slower buring powder to be less for similar reasons.

Phil