All of the physical components: peak pressure, average pressure, base area (bore) of the ejecta, weight of the ejecta, and time, are all inseparable pieces to the acceleration of the ejecta... and which leads to the opposite reaction, acceleration of the firearm.
At peak pressure, the ejecta will be accelerating at its highest rate (g-force), unless it's constrained from movement. The peak acceleration of the ejecta will coincide with the peak acceleration of the firearm in the opposite direction (not the peak velocity of the firearm). Those recoil charts in Drew's reference tell a lot.
