Walt, Adding anything on top of the powder charge changes the internal volume of the case and will increase pressure for any given powder charge. In normal, center fire cases using smokeless powder, with a powder that is appropriate to the cartridge, no filler is needed, and may in fact reduce accuracy.
Black Powder cartridges are a different kettle of fish...in any event, consult a good manual... one updated in the last ten years or so, and choose a good powder...Lyman's manuals have always indicated the best powders for velocity and/or accuracy as a part of their write up....I'd suggest starting there. Forget the fillers, they're just not used anymore.
And welcome back to the fold....Rod
Our Flag does not fly because the wind blows against it,
It proudly waves because a soldier's dying breath blows upon it.
USAF Forward Air Controller, 5th Spl Forces,
An Loc, lll Corps, RVN, 69-70, Vietnam Vet '69-'73