Assuming nominal bore dias (.775/.729)² = 1.13 or 13% more volume for the 10ga. The quick & simple shortcut for volume of differing bore dias.
Yes I am well aware of the low pressure loads listed in manuals. Lets just say that from personal experience on a day I was breaking ice into a duck swamp (pre lead ban days) I experienced failure while using a load of 7625 taken directly from the then DuPont loading guide @ close to 7K psi, nowhere near that 5K one. I had worked too hard getting into that swamp to have it totally ruined by inadequate combustion, so "NEVER MORE" for this ol Boy. I will no longer load any powder slower than PB/Greendot etc lower than 7500psi & even then only with the hotter primers. These powders were designed for heavy shot loads at normal velocities with normal pressures, NOT for low pressure combustion. They get that quality by making them "Harder" to burn.
I can only state my experience, everyone has to decide for them selves, but if you are experiencing the differences you mention my guess is you are on the Ragged edge & any number of factors may give you a squib.
