If the shooter is sensing buttstock acceleration as felt recoil, as opposed to total recoil, then there could be the following issue. The payload is constant in weight, so the rearward acceleration of the gun will be basically (neglecting the powder gas portion of the acceleration) proportional to the forward acceleration of the payload. Acceleration will be basically proportional to pressure except that there may be a slight additional acceleration in the forcing cone if it is sized and shaped such as to have some venturi effect on the shot charge. Also, per Bell, there might be enough pressure rise to increas acceleration enough for a shooter very sensitive to acceleration to sense it as increased recoil.
I would not totally discount the possibility, but would add that my experience, which seems to match general experience, is that it is not a factor for me.
