300 gr Hornady RN Interlock bullets with 68.0 grs of IMR-4064 is a very accurate load in my Ruger M77 Magnum in .375 H&H. (I haven't tried the new 300 gr RN Interbonds yet.)

Personally, IMHO, if you're getting 10 inch groups at 150 yds I think the problem lies elsewhere besides the bullets. A broken scope, loose scope mounts, action bedding, etc. could be some of the causes. Have you gotten decent groups with this rifle using other bullets?

-Bob F.