Hello,
There is a product called PuffLon.that is a very fine powder just designed to be used as a cartridge filler. It works fine.I use my Lee powder measure kit spoons to get the exact same amount in each cartridge.I have never heard of any problem using this stuff.
I did discover that,if you compress it,it will form a solid ball,over time,just like any cereal(cream o wheat,grits, etc).That's why I use the measuring spoons.I fill the case,without compressing the powder.
However,90% of my rifle cast bullet loads use Dacron.I never found a use for fillers in pistol loads,and the only filler in the litle Hornet cases are for fire forming to my Kilbourne Hornet rifle.
In your casse,I recommend the Puflon.
Frank