I'm afraid I can't help you with the AR-series powders, but don't give up. The 458 Win Mag can be loaded with 405 gr cast bullets quite nicely. I use IMR 3031 and 4198 with standard primers for loads around 1800 fps. For even lighter loads, I've used Alliant 2400 to get 1400 fps.

Look on a burn-rate chart for powders in this range.