When I competed with roundball rifles in NMLRA competition I would sift my black powder that I would use before a match, for my cross-sticks rifle and my lightweight benchrest rifle. That got rid of the fines and the extra large, coarse particles. I saved them, of course. You would be amazed at the amount of fines I would get out of FFg. I had three sieves that I used to do this procedure, the sifting. I would only do a small amount at the time, then can it and seal it up. This was to lower the chances of an unexpected ignition while I was sifting and sorting. The difference between group sizes with my rifles was pretty impressive when I used sifted powder, as opposed to straight out of the can.
I'll look and see. I may have a can of FFFFg. If I do I have absolutely no use for it, and I'll let you know, Keith (67galaxie).