I have a Liberty M77RS that caused me all kinds of trouble trying to find a good load for it. It has a long throat, of course. I finally tried Bob Hagel's load of a compressed load of MRP and a 140gr Nosler Partition. My rifle shoots that load MOA at 2895fps. The load is listed in his "Game Loads..." book.
That load is too hot in my 1A 7x57 which has a shorter throat.
I believe that, in rifles with long throats, a flat based bullet will be more accurate.
Based on what I have read on this forum and on other forums, many guys are having good success with the Hornady 154gr flat base bullet. You might try that one.
I considered the flat based possibility. I did fire an ok group with factory 140 core lokt, which had been the only flat based bullet put down range. I then loaded up some 139 hornady flat based bullets and didn't see much improvement. I didn't play with powders much with the flat based bullets either, however. I'll keep plugging away. Eventually, I'm sure I'll hit pay dirt. Thanks for all the suggestions.