You might also check the Accurate #2 book. They have cast bullet loads for the .458 Winchester Mag in 400gr., 475gr. and the 500gr. roundnose bullet. Also some with gas checks. The 500gr. roundnose shows a starting load of 42.3grs. of 5744 for 1672fps., max load of 47.0grs. at 1901fps. with 49,900 CUP. I've always liked that 5744 powder. At 1901fps., that's moving pretty good for a 500gr. cast bullet. Better have some good lube or a hard cast bullet.