The Area of Bore #1-12b is .4176 sq in or .4176 cubic inches per linear inch after chamber. The area of bore #2-10b is .4719 sq in/ .4719 cubic Inch per linear inch after chamber cone. Vol 2 is 13% larger / linear inch than Vol 1 therefore same gas volume would exhibit 13% less pressure. Thats the physics of it as it relates to gases. What may in fact be occurring is that the 12 b load/bore combination is at the low end of critical pressure(temp) to effect efficient combustion of the gases. Decreasing the pressure a further 10% or more may kick it over the edge resulting in incomplete combustion and a significant drop in pressure. Should you wish to attempt regain performance do not increase powder in anything more than 1 grain increments as pressure may rise quite quickly once the point of efficient combustion kicks in.
