I don't know how many times this has come up in my club!!! .
I've ben shooting BP revolvers even before they took my cartridge pistols off me. I've experimented with all sorts of fillers, powders, wads, ball and conical andcome to a firm conclusion.
Don't use an inert filler. So many conditions, as mentioned above, affect a filler. Moisture, density and quality from batch to batch all destroy the main factor for accuracy..... consistancy.
Some will say that a filler has to be used to seat the ball as close to the chamber mouth as possible....... again, I say no. In the experiments I did using differnt seating depths, it made no difference. The ball still travels down the "tube" of the chamber whether a tenth or a quarter of an inch. It has to cross the cylinder gap and then enter the forcing cone. The forcing cone is what aligns the ball down the barrel not the seating depth. As long as your revolver is timed correctly there's no problem.
Seating depth in revolvers is aptly demonstrated with the .38spl or .357 revolver. Generally, the most accurate round in these revolvers is the 140 wad cutter. That bullet is loaded level with the case mouth! How much chamber does that travel before entering the forcing cone?
Use an accurate flask/spout or indivdually weigh your charges. If using cast balls with a nib from the sprue plate, orientate the nib so it's at the front. Use a felt wad between the ball and powder and remeber the ignition time for BP and it's substitutes is slower than cartridges,
so concentrate on correct hold, sight picture, trigger press and follow through. Your results will be shown on your target.....