I've used W760 in the .358 NM and the .375H&H is all, Mick.

I merely noticed the normal .338 mag. slower burning powders would be running out of case room with the lighter bullets - according to the manuals. It's common to go to a faster powder for very lightweight bullets, slower powders for heavier ones. The .338 is ballanced well enough that it can easily use the intermediate powders like Varget and H4895 - with 165 through 185gr. "X" bullets.