I wouldn't count on lengthening a forcing cone reducing max pressure to any extent. Pressure has peaked & starting to fall by the time the shot gets into the cone. To effect pressure by much the chamber is going to have to be so short as to actually retatrd the opening of the crimp.

Every shell out of a single box of shells fired through the same gun will give a slightly different pressure reading. I Highly suspect had Bell run a lot larger sampling those pressures would have tended to be even more equal than he showed.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra