I have an RMC flinter that I have two barrels for. They are Green Mountain .50 cal. 1:28 twist in 24" and 28" lengths. They are the same barrel that you have. With my 24" barrel the groups start to open up after 80 grains of Goex 3f. The barrel really likes the Barnes Spit-fire 245 grainers, 385 Hornady Great Plains and Hornady xtp's in both .452 300 and 250 grains. I use mmp short black sabots for the Hornadys and the supplied yellow sabot with the Barnes. The Barnes are so accurate that the bullets all touch inside a 1" square at 50 yards. It will group well with 90 grains too but the groups are larger and it's not burning it all so why use that much.

I believe the Swiss brand 3f powder will give you the most velocity and accuracy from that 21" barrel and would limit my charges to around 70 grains. Your groups will shrink a great deal if you lower that charge a little bit; it will still have plenty of power out to 100 yards.