Bob-
I'm going to put this in a nutshell for you.
Don't think about it so much.
Use the filler.
You're risking ruining your rifle by way of ringing the chambers without a full case of powder/filler/bullet.
13 or 14 grains of dacron is your friend.
I use 21 grains in my 500/450 3-1/2" Magnum BPE over a big charge of Varget.
Have you read Graeme Wright's book on loading, handling, and shooting the double rifle?
He has extensive pressure testing results from running handloads and factory ammunition at a British Proof House.
Cheers
Tinker
