The powder that most often in most rifles gives the combination of highest velocity and accuracy is Reloader 15, especially if you don't like heavily compressed loads.
Varget and can also give very good accuracy and velocity but can also be very fussy or erratic for accuracy.
If you are happy with loads that basically duplicate factory ballistics (actual) IMR 4064 generally gives good accuracy results with about 68 grains for 300s and 71 grains for 270s.
In my experience it is rare for an accurate 375 not to give very good accuracy with 78 grains of 760 and 300 Hornady round nose and also 300 Sierra boat tails. In fact for me, a 375 that won't shoot well with these loadsmakes me immediately suspect a rifle, scopeor mount problem.
Like all calibers of course, each rifle is an individual, but the above seems to be generally the case when used across a large number of rifles.
Mike
IMR 4064 and Varget will give OK results (frequently excellent accuracy) with the 270 gr bullet, but you will sacrifice some velocity. If you use those powders for the 300 gr. bullet you will sacrifice a lot of velocity, compared to what you can get with H 4350.
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Ray Atkinson
260 Nosler 76.5 RL-15 2700??
285 Speer,Northfork & GS solids 74 RL15 2641 FPS chron.
300 Woodleighs, soft or solids 72 RL-15 2585 FPS chrono. (2645 in my 26" barrel)
Some other good loads are:
285 Northfork 67 IMR-4064 2560 chron.
285 Northfork 77 WW760
300 Nosler 76.5 IMR-4350
350 Woodleighs
soft and solids 66 grs. IMR-4320 2440
67 grs. RL-15 2467
these last two have got to be a great load and bullet for the 375, Perhaps equal or near equal to the 416 Rem. an Rigby..Time will tell. I know they sure are accurate bullets in my Winchester M-70 pre 64 and in my Holland & Holland.
These loads are mild and shoot well for me and I'm not into squeezing out the last drop of accuracy on Buffalo rifles but all these loads shoot an inch in my present rifle.
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Ray Atkinson