Hornady gets 2400 fps with a 66.8 grain charge of Reloder 15 and a 300 grain bullet. They max out at 70.1 grains with their popular 300 grain bullets. If you are shooting solid bullets, you do not want more than 2400 fps for several reasons: Over penetration, shooting through several buffalo with the same shot. No doubt wounding one or two, creating a very dangerous situation. Many solids of conventional design rivet, and thus fail, at much over 2400 fps. It is exceedingly difficult to get solids and softs to hit the same point of impact at velocities over a modest speed for a .375 H&H. My own .375 H&H's respond well to the Reloder 15 loads when I am using solids and softs. One of the various 4350's would be appropriate if you were going to drive 300 grain softs of premium construction at maximum velocities. Winchester 760, Hodgdon 414, and Accurate 2700 ball powders(they are the same powder made in the same plant) are also favoured for throwing 300 grain slugs very fast and accurately.

I have never seen Reloder 22 used in a .375 H&H, let alone Reloder 25 or the VihtaVuori N560.