Playing with a friends 375 remington ultra mag we settled on Aliant reloder 25.
My winmag like's IMR 4831 with 180 grain barnes tsx. Have not tried the sc.
The 375 H&H has a radical different case geometry from the 375 Remington Ultramag
The powders, for the H&H cases are generally two or three levels faster than the extra slow powders for the ultra mag series. While the H&H cases thrive on medium, slow powders, such as Varget H 4350 H 4895.etc.
The ultra mag group uses the very slow powders, such as H 1000 and retumho H870 and even slower.
Generally speaking the larger the case volume compared to the bore diameter, the slower, the powder Hass to be conversely the smaller case capacity compared to bore diameter the faster the powder has to be.
I’ve loaded 300 win mag 300 Weatherby 338 win and 300 ultra mag. I have not loaded for the 375 H&H but a quick check in a loading manual I will show you powders up to cfe223 for the.375H&H.
Once again, generally speaking, the heavier, bullet requires a slower powder and the lighter, the bullet, the faster, the powder in any given chamber and bore diameter. This last little bit of info will help in the final selection of powder, given the choices for that caliber and cartridge and bullet weight.