If you have not tried handloading for it, give it a try. I have a M77 in 7MM Mag that does not like factory ammunition. When I purchased it new I also bought ammunition from Federal, Winchester, and Remington. I was lucky to get a 3-inch group out of it.
Reloading for it resolved the accuracy problem. Three case filling powders have made the difference, H4831, AA8700, and H870. Using the listed powder I have had very good results with bullets from Remington, Hornaday, Nosler, and Speer.
In the case of the .308 I would try IMR4895, IMR4064, or IMR4320. All three powders will give you a load density over 95 percent. In the case of my M77 load density was the key to success. When I bought bulk Remington bullets and created high density loads my groups tighten up and I changed my mind about get rid of the rifle. When I started my pre-season practice sessions last Spring I was again reminded of the truth I discovered over twenty years ago. My handloaded Remington CorLoks and Speer Mag-Tips created tight groups. I then open a new box of Winchester factory ammo and the groups opened up. I decided I would shot up the two boxes of factory ammo for practice and reload the brass.
I am aware of the same problem with a couple other M77's built back in the early 70's. The guy who owned the rifles sold them to a couple of brothers who I hunted with. Reload ammuniton resolved the problem. A strange one at the that.
Hornaday Light Magnum and Federal High Energy loads are compressed and may serve the same purpose.