.375 Magnum Ackley improved
235 gr..............82.0 to 86.0 gr( max) Imr4064.@3060 fps.
270 gr..............80.0 to 81.0 gr( max) Imr4064. @2830 fps.
300 gr..............78.0 (max) gr Imr4064 @2630 fps
300 gr ............ 85.0 gr Imr4350 to 88.0 gr (max) @ 2740 fps
350 gr...............88.0gr (max) 4831 (Not specified if Imr or Hodgdon)
2650 fps.
.......................Ackley Handbook for reloaders vol #1.......
Ackley manual provides exact same powder and load data and velocities as .375 Weatherby ,as stated by Rembo..
hs4570.....your mileage may vary