An easy shooting, low pressure load with a jacketed bullet, is 42.0gr. H335 with a 220gr. Hornady .375' bullet. The velocity will be in the 1,900fps to 2,000fps range and was very accurate in my .375 Win. M94.

Ken Waters did a .303/.375 or .375/.303 some time ago, calling it the .375Rimmed iirc.