Optimum bullet weight is determined by barrel ROT. A 1:9 barrel should be able to handle everything up to about 75GR.

I shoot 62gr in both my 1:9 and my 1:8 barrels.

Regarding stabilization - a study was conducted (Army or Marines, I don't remember) that found the vast majority of infantry engagements happened with 75 yards (or less) separating opposing forces. If your standard engagement ammo stabilizes properly out to 100 yards, you're gold. If you're interested in longer ranges, get a faster barrel (1:8 or 1:7) and use heavier ammo (70+).

FWIW, the steel core stuff is used specifically for its penetration capabilities at close ranges (out to 100 yards), such as going through glass windshields, thin walls, and even steel plate, and not being affected by incidental contact with obstructions in the line of sight.

For practice, I use Tula 62gr, but my "ready" mags are filled with 855 steel core, and I have no problems with accuracy with either one, regardless of the rifle I use it in.

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*Aim small, miss small...