Well, that's just a Wicked Awsome rifle my friend!
Load 70-72 grains of RL-15 with 270 grain Hornadys. Should be fairly mild to shoot and will poke a hole through a deer with minimal meat trauma. Probably in the 2500-2600fps range.
You could shoot the 220's out of it but I never liked those flat nose bullets much. The beauty of the 270's is that they should hit dead on with the 270 Aframes. So for fooling around you can shoot inexpensive and move to the Aframes for hunting. We are loading the Aframes with 75 grains and getting right at 2800fps with surgical precision. here is the trajectory:
100 meters +2.5"
250 meters "0"
300 meters -5"
350 meters -11"
This means that a spine hold on big game animal out to 350 yards is still dead with that big wind bucking bullet.
Another well accepted fact where the 375HH is concerned is the way the 300's and 270s shoot from just about 100% of the guns made. I have not seen one that did not do this.
With the above zero point using the 270's the 300's should fall into a zero at 90-110 yards. This allows plains game hunting with the magnificent 270 A frame soft points and with a simple switch to the 300's you are ready for anything else. Truth is you only really need the 300's for the huge game like Elephant, Rhino, Hippo, and maybe even the Buffalo but softs for the buff rather then the solids used with the thick skinned game.