Whether it is tones, CUP, or PSI – the instrumentation is designed to generate a reproducible value representing “pressure” at a given velocity. That value could be anything including bananas, as long as it is consistent and linear.

The area under the curve and shape of the curve depend on the burn rate of the propellant, and can only be explained with black magic.

However; in Graeme Wrights book he touches on medium versus slow powered. He suggests that slow powders may carry too much pressure in the thin area of the tubes and that medium power like R15 reach peak pressure in the first 6 to 8 inches where the barrels are strongest. Makes sense to me