I haven't shot cast in the .375 or the .458 WM, but have shot some in .38-55 and a lot in various .45-70s. I did have a Ruger No. 1 in .416 Rigby several years ago, and got 1.2-inch groups at 100 yards using the RCBS .365-grain gas check and something like 51 grains XMP5744 for about 2,000 fps. Very pleasant to shoot.