Gentlemen,
I use fillers virtually all the time for shooting the old BP cases with smokeless powder: Thousands and thousands of rounds.
The prime filler for me is T/P, preferablely Charmin, chosen for not for its softness but for its maximum "loft".
A fellow has to insert the T/P filler into each case carefully such that it will fill the void, preform its function but not become, as it were, a projectile within the case itself. The cause of chamber rings is when the filler or wad or whatever is "fired" down the case, hits the stationary bullet base and an instantaneous pressure wave expands the steel of the barrel. Thus the filler has to be placed and remain in place right at the base of the bullet. You must really fill the case with a compressed amount of filler such as is done when using cereal fillers so that no mixing can happen or make sure the light fluffy fillers are pushed into the case by the bullet as it is seated so that their bases are ALWAYS in contact with the filler material.
Good luck,
Forrest