I recently came by some fine-grained Goex black powder, around 2FG or finer. A pyro friend and I were hashing over what the stuff could be used for, and he told me about a great crossette comet break mix, which uses fine-grained black powder and flash powder. If you’re unfamiliar with crossette comets, they are comets that, after rising or igniting, are seen to split into several pieces in the sky.
They can be constructed so that they split into either a precise number of pieces or into a random number of pieces. The comets are made with a cavity in their center. The cavity is then filled or partially filled with an explosive charge (the "break"). As the comets burn, the fire eventually reaches this center charge and splits the comets into their pieces, creating the crossette effect.
My friend uses a break charge that is a mix of flash powder and fine black powder, 10 parts flash to 8 parts fine grained black powder (7FA or 4FG or something in between). The reason for the black powder in the charge is to add some "fluff" to the flash powder. I have heard of people adding bran, sawdust, rice hulls, Cab-O-Sil, and other such agents to flash powder to aerate it, and thus speed up the burn/explosion, but this is the first time I had ever heard of black powder being used. Makes sense when you think about it, back powder will burn quickly, the other agents won't.
My friend told me that this break mix works so well in his 1-1/4 inch crossette comets that he only uses about half of the volume of a .22-short caliber shell casing! The cavity in the comet is perhaps 3/8-inch diameter by 1/2 inch deep. (The best instructions I have seen for making crossettes can be found in the Fulcanelli article in Pyrotechnica IX, (BK0111)).