Use FFFg in your percussion guns. It will outperform the FFg powder. It has nothing to do with burn rate. It has to do with consistency of burning, and the lower SDV that occurs. Use a compressed load, and the more consistent you are in compressing the powder, the lower the SDV.
I recommend using your barrel like a drop tube, and hold the barrel straight up when pouring in the powder charge. That gives you gravity pulling down the powder the entire length of the barrel. From experiments down with this technique compared to using a ram on a reloading press and BP cartridges, the drop tube compresses the powder better than anything that can be done mechanically, short of a hydraulic press. When you run the PRB down on the powder, seat it firmly, but try to use consistent pressure.Changes in seating pressure can be easily seen on a Chronograph.
The secret is not to use a jag that has the wrong shaped cup to its face, so it deforms the RB in the barrel, and not to use so much pressure in seating the ball that YOU DO DEFORM the ball. The accuracy is much better with a true Round Ball, and not one that is deformed.