Check out this link: P-Max black powder internal ballisticsGood you got that figured , but my 12 inch 1-28 twist muzzy with off shore (highly regarded) barrel shooting 80-90 gr OE 3 F or 75 gr weighed sub (BH 209)gets a Great Plains 50 cal staying in 1/2 a pie plate @ 200 yds as long as I can stand the cold off a bench .And not for nothing don't say any thing about wasted powder ! That would start another discussion with the pristine 4inch of white snow and my chrono !!/Ed
This is a fairly sophisticated velocity calculator for black powder. I am curious to see the ballpark difference between your 80 - 90 grain charge of OE with the GP bullet vs. my 30 grain charge with round ball. I'd do it myself, but I don't know your bullet weight. 12" barrels are fun to play with.