Not really giving you an answer but related to the topic.

I've tried cast lead projectiles in my 375H&H (rifle didn't shoot the ones I tried particularly well). When I first started using them I was using standard LR primers with about 30gn AR2205 and no filler - this resulted in hang-fires. I then tried using half a cotton 'ball' as a filler. This cured the hang-fire but every shot resulted in a cloud of cotton wafting around and sometimes back onto the bench onto other range users (who wondered what was going on the first time it happened to them - it's amazing the amount of fibrous material in half a cotton ball).

Ultimately I ended up using magnum primers and no filler.