For the .223....
According to Sierra at least 1-10 twist for the 69gn SMK bullets. 1-8 twist for the 77SMK. Don't know why the 77gn works so well in my Axis, but, it does. Could be that shooting it from the 16" (or shorter) barrel of the typical AR does not give it the extra velocity that I get from the 22" barrel. Mine exit at 2850fps.
Twist rate for the 55gn bullets is more like 1-12 if you look at the early AR15/M16.
Except for some very specific aerodynamic issues, over stabilizing a bullet is not 'harmful' until you get to very high velocities with thin jacket bullets.
FWIW, the Hornady 68gn HPBT match bullet was not very accurate at all in my rifle. Others have had the same experience. Nosler 69 and 77gn bullets were good, probably 0.1 or 0.2 worse than the SMK's. The 75 ELD was also a good bullet.
Hornady makes a 52gn ELD and 55gn VMax that I would try if I wanted a lighter bullet. Or the SMK.
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