A nice mild load that has given me incredible accuracy in the 375 Wby is 13.0 gr. Green Dot. It will keep most shots touching at 100M if the wind's not too strong. Any of the faster flake powders like Red Dot, Green Dot, 700X, Unique, etc will work without fillers. Also a case full of very slow powder will work well too. I use a lot of IMR 7383 to H870 range powders for cast use, 870 will get you maybe 1900 fps with a case full and a mag primer, usually very good accuracy, but burns dirty since it's not building enough pressure to burn clean. As long as your bullet fits the rifle's throat, is hard enough, and is using good lube you should be very impressed with what a 375 will do with cast bullets!