Excellent Discussion;
Now let me throw another pea or two in the pot. Assume the chamber insert is bored to exact dimensions of a regular 12ga chamber (it should be extremely close) then no gas volume has changed untill the base of the wad clears the end of the case. As max chamber pressure is normally reached & started to fall,prior to this point, no significant change in max pressure would be expected.
From the above quoted formula;
Quote:
Boyles/Charles law [discovered 1662] can be used to "ESTIMATE" the pressure drop as the charge leaves the 12 G. gaugemate and enters the forcing cone and 10G barrel.Boyles law states tha:P1xV1=P2xV2. This formulae can be transposed to:
P2 = [P1xV1]divided by V2

Note, This formula applies to "Total Volume" of the area containing the burning gasses. This includes the volume of the cartridge case as well as the bore it is entering. Thus iregardless of what the dia of the 10ga bore id in comparsion to that of a 12ga at the same point whether it be 6%, 13% or someother % that increase in volume is not immediately reached upon the charge entering the larger bore. The obturating cup of the wad would however immediately spring outward from the applied pressure to immediately give a larger area for the gas to push upon. Thus for at least some short distance after exiting the shell case the area to which force is applied would be greater than would be the falling off of pressure in a comparsion between the two gauges. Acceleration could then be expected to be greater in the larger bore, considering of course an adequate bore seal of the wads. "IF" that does not happen & blow-by occurs then all goes out the window.
Personally I have no inclination to try & calculate all the volumes necessary to come up with exactitudes. This would have to include inside case volume, volume of the forcing cones, & bore volumes. To get absolute definitve answers one would then have to figure changes in applied friction as well as change of the powder burn rate caused by the changing volumes.
Too much for this Ol Boy.

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