Please don't use anything but real black powder.

A good friend of mine with a made for India (gold script with Maharahja's name) DR in .500(Blissett I think but not sure), used the 450gr. Lyman bullet well. I think he had a machinist enlarge the grooves in the mould by a few thousanths due to the mould casting undersized for his rifle. Most of the those moulds, cast .515". Both mine cast this size - 1 Lyman and 1 Lee.

Each barrel would shoot into 1 1/2" at 100 yards, but 2" between group edges IIRC (30 years ago), which made for a rather wide but short 5" group.