The 378 and 460 load down beautifully to 375 H&H and 458. In fact in my last 460 one of the most accurate loads was 85 grains of IMR 4064 and 500 grain Hornady for just over 2000 f/s. Another great load was 28 grains of Hi Score 700x and the 400 grain Speer. Did just over 1300 f/s and would hold 1 inch groups. No filler for either load.

However, where you run into a problem (in Asutralia but not America) is with loads that sit between 458 and those 1300 f/s loads. The culprit is the very long freebore and big case capacity which causes hangfires. You have a powder which I think is Accurate Arms 5544 (not sure of the number) and that powder is about 4227 burn rate but is bulky and flaky like shot gun powder and so solves the problem.

Mike