There are two things that I do when I want to load and shoot reduced loads in my M.70/.375 H&H.
The first is not really a reduced load but a lighter bullet. I use a .375/220 grain bullet designed for the Winchester .375 along with 40 grains of AA5744.
The second is that same powder (AA5744) - 35 grains - with 270 grainers.
Ive used TrailBoss with cast successfully in all my bottleneck cartridges, though I dont have the 375H&H; I load the TB to the base of the bottleneck, ie, just where it starts to taper to the neck from the case body. Works for me, you proceed with the 'work up a load' cautions as usual.
Im sure this would work with jacketed bullets though those designed to do their work at higher velo's may expand at lower velo. The good thing is you do have some bullet selection for this caliber unlike the typically 'wide-bored' 38-55 that the typical .375 bullet is too small for.
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