No need for filler. I have loaded 45/70 kid rounds for my grand kids. About 20gr black/Pyrodex under a round ball. Worked well out doors. Pick a ball that will fit snug in the case and press it all the way down on the powder. No air gap no filler no crimp. Just like loading a Cap&Ball revolver. You may want to lube the ball with something like Lee Alox or do what I did and top it with a grease cookie. For indoor use filler or grease will make a VERY big mess. I will look up the ball size I used later. ---- Might want to try rubber bullets with no powder. I use them in 45 Colt for revolvers in the shop.