I'm working on such a load right now. I want something that will give adequate penetration on the neck bones of an elk. All of my initial testing is done in a 4 5/8" Blackhawk.
The load will only be used to dispatch an animal that's down. Two years ago, I hit a cow elk high through the shoulders and required another shot. While my .375 H&H certainly did the trick, it wasn't a result for pictures.
Im using a 210gr Penn TC. I found that WW296 was difficult to use as pressures ran too high. Strangely starting in the middle of all of the recommended loads I could find pressures were higher than I wanted to stuff in my M19. Less powder produced higher pressures, more powder even higher.
All Federal brass trimmed to minimum specs with a Lee. Win Mag Pistol primers with 296, Std Pistol primers with Alliant powders.
5.0gr of Herco was safe enough, but velocity in my 2.5" M19 was 840fps, my 4" M66 was a tad over 900. 5.5gr and pressures increase fast so be careful, I currently have 7.0gr of Blue Dot loaded up and will run that over the screens soon.
I.m hoping to get 950fps with modertate pressures.