Since you have the rifle and are probably not looking to rebarrel for competition, but to simply tighten your groups, then you should try something in a spbt or hpbt. 150 to 180 gr should do fine with your current twist. You really cant expect 1/4 MOA 5 shot groups with a factory rifle even with custom reloads. Just remember, each rifle is different. You can have 2 identical rifles 1 serial number apart and they shoot differently. For accuracy, my old M77R likes the Sierra Matchkings the best.
No such thing as a stupid question. What is stupid is not asking it.