When most of us speak of pressure, we're speaking of the "spike" and the peak pressure produced. Peak pressure certainly has very little correlation with ballistics or recoil. Pipes mentions pressure "under the curve". I'd think you could average the plot for sustained pressure over time and that psi number would be the same for all powders and powder charges producing the same ballistics for a given payload. I continue to think that the time to "peak" is bound by necessity to the inertia of the payload.

jack