I just began shooting a 10 gauge Damascus barrel SxS with 12 gauge Gauge-Mates (short sleeves).

Clearly the pressure (and "power") is significantly less than when firing the same load in a 12 gauge barrel. I'd like to understand the mathematics of calculating the likely pressure reduction percentage, due to the more spacious 10 gauge bore. I think it requires a calculation of the volume difference?.

If every other aspect remains the same, what is the likely pressure reduction of shooting a 12 gauge shotshell in a 10 gauge bore?

If someone can provide the calculation/answer, I would have some idea of how much to increase the powder charge in order to provide a similar pressure load; or a desired muzzle velocity. I'm using IMR #7625 powder in the 5,000 fps range.

Thanks for your assistance...

gold40