I would look at the loads for Alliant Power Pro 300-MP. This is a new powder from Alliant especially made for large magnum pistols like the .454 Casul. Here's one for the Speer 300g GDHP: Min OAL: 1.75, Barrel length: 7.5", Primer-Federal 205M, Powder-300MP; Grains-31; Velocity-1741 fps.