The most common fillers for reduced loads in cap & ball pistols are corn meal, grits, wheat bran, and Cream O' Wheat cereal. I once witnessed a serious competitor using plastic shot buffer, so there are probably several more useful fillers out there. Corn meal is compressible and forgives a moderate amount of variation in loading. So too are wheat bran and grits. Cream o wheat does not compress and must be measured to avoid a ball standing proud so the cylinder won't turn. The compressability of corn meal and the other fillers is often mentioned in recommending them. However, the inability to compress may be a significant reason to use Cream O' Wheat(COW). Being able to provide the same compression on each powder charge with filler and ball will result in more uniform burning and velocity spread. My carefully protected Chrony has shown that a load of 27 grains of Goex FFFg, 13 grains (measure) of COW, and .454 round ball has the same velocity (880fps) as 37 grains of FFFg and the same ball. Sure, the COW adds its weight to the projectile for hotter powder burn but I have not seen this with corn meal or other compressible fillers. Accuracy is much better as well, particularly where the chambers are a bit under groove diameter like so many Italian replicas have.