I've been given some 500gr cast loads (Hard lead, I'm told) and wondered where I should start with a load for my 458WM? Ideally, I'd like to make power for Group 2 at the Big Bore club (Meaning over 3900ft/lb) which by my calcs leaves me needing around 1900fps from a 500gr bullet.

Lyman's cast manual shows a 475gr at 1650 for a max load, so I'm wondering if I'm after the impossible?