I could not count the number of 375s I have owned or been directly involved with.

The first load I try in any 375 is 78 grains of W760 and 300 grain Hornady round nose. Either Fed 210 0r 215 are about equal. If the rifle does not shoot this load then the odds are extremely high that the rifle has problems, whether barrel, bedding, scope/mounts. The 300 grain Sierra has also been good but I have not tried it in nearly as many rifles and the Hornady.