If I had to venture a guess about your inconsistancies with these loads I would say that the .600 RB would be the issue. It is quite a bit under sized for the bore and is probably letting your gas seal blow out or pressure to blow by it since it doesn't present a "flat" surface against the seal like a load of shot and it is too far away from the inside of the bore.
A .690 like the feller posting before you would fix that.
The .735 you spoke of would probably require a this brass hull like the CBC/Magtech.