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.
I would tend to agree . . . except for the fact that I don't believe he ever recorded an INCREASE in pressure as a result of a lengthened cone. There was always some decrease. I do agree that in order to determine how much of a decrease one is likely to get, you'd need to fire several shells (all the same loads) through the unaltered cone, then do the same through the altered cone and take an average.
