He stated that every inch multiplied by two the pressure dropped in half.
This statement would be true I believe if the volume exactly doubled "& IF" the temper stayed constant. In real life of course neither of these are true. The internal diameter of the hull will normally be of a larger diameter than the bore, plus the forcing cone will have more volume than the same length of bore. Although I do not have the means of measuring them I am quite certain the temperatures of the burning & expanding gases are not constant for the length of the bore. Pressure is relative to both volume & temperature.
Max pressure is of course reached at the point where increasing volume of the chamber exceeds the expansion of the burning gases, but all the powder has not been burned at that exact point. The pressure therefore would not be expected to have fallen to half of the max at a point where twice the volume was reached from the point where the max occured. A look at recorded pressure curves of various burning rate powders will quickly show these exact ratios simply do not exist in the real world.
