I put a Mark X .375 H&H barreled action in a stock for a client 20 years ago and it shot groups under 1 inch at 100 yards for 3 shots.
Loads for the Mark X:
xx grains H4895, 235 grain Speer
xx grains 3031, 235 grain Speer
My Enfield .375 with a Shaw barrel will group in the .6ths inch range at 100 yards with jacketed bullets and shoots the following cast bullet load well.
270 grain Eaton Cast Bullet xx grains 3031 velocity 2193 FPS Energy 2901 FP group size .910 inch for 3 shots at 100 yds.
Accuracy load:
235 grain Speer xx grains H4895 velocity 2427 Energy 3072 group size .608 inch for 3 shots at 100 yds
Highest energy load:
270 grain Hornady, xx grains H4895, velocity, 2621 FPS, Energy 4118, this is from Ken Waters Pet Loads, and it exceeds the Hodgdon manual by 2 grains.