If the internal barrel volume of the 10 ga. is 20% greater than the 12 gauge, will the pressure therefore be 20% less?
Considering nominal dias of .775 & .729 bbl volume of the 10ga is 13% greater than that of a 12ga. Volume ratio of circles = D²/d² thus .775²/.729² = 1.13 or 13% greater for the 10ga. Real;ize of course while the 12ga shell is inside the chamber reducer the dia of the inside will be essentially the same as it would be in a 12ga chamber. You would thus have essentially the same "Chamber Pressure, though you would have the benefit of the insert walls & would be operating on a smaller dia than a 10ga shell. The increased volume would come into play only after the charge had cleared the shell & entered the bore proper.
