When I lived in Alaska I had a 375 H&H with a 24" barrel that I could not get up to speed with 3 different powders. IMR-4350 I couldn't even get enough in the case to make a 270 grain run as fasts as a 300 grainers advertised speed.
I bought a copy of Ken Waters Pet Loads and the Powder that he used that got the highest velocity was W-760. I tried 760 and low and behold I was able to get this rifle to run at advertised velocity
Ken was no dummy that is for sure and for certain
