H4831SC is a good place to start. It's one of the better powders to use in the 300 Win. Mag. As far as the best powder I think if you took a poll that RL22 would probably win out especially with 180 gr. bullets. If you like the 200's you can try RL22, RL25, & H1000. Out of a 26" barrel 300 I reached a little over 2900 fps with good accuracy with RL25. This is one particular rifle I sold an have regretted it ever since.