Hi Joel.
Graeme Wright's book, "Shooting the British Double Rifle" has been spot on every time for me on other calibers. He says with a 350 jacketed bullet, 70 grns. Varget, shot pellet buffer for filler touched off with a federal 215 primer.1950 FPS.
I have a Hollis full Nitro 500/450 if you want my recipe... wink