Well my figures were not meant to be Absolute" but just a ball park figure. I picked up one each 12ga shell & .45-70 cartridge. the head of the 12ga miked @ .808" & the .45-70 @ .501". The area of a circle of .808" is .513 Sq In while that of a .501" circle is .197 Sq In. .513 ÷ .197 = 2.604 so when I said about 2˝ times I was actually being a bit conservative. To be absolute & technically correct I believe one would have to take the inside diameter of the case as the walls & rim area would add no thrust, only that upon which the pressure actually reacts with.
One would then have to deduct the retardation given by the "Grip" the case has on the chamber walls. Actually as I said I totally approved of your choice, but based upon all the other factors you mentioned, not because of the difference im max chamber pressure.
