I have a CZ527 in 6mmAR that was built using a Shilen 10 twist barrel. I can push 65 gr. Hornady V-max bullets at ~3400 fps with AA2200 with less than 1/2 MOA accuracy. It also shoots the Nosler 90 gr. AB bullet at ~2900 should I need to go after something bigger with it. The newer 6.5 Grendel/7.62x39 magazines feed perfectly, while the older 7.62x39 magazines don't.
If you're going to build a 6mmAR, I'd start with a 6.5 Grendel. See how the Grendel shoots for you, as you could always make your rifle into a switch barrel rifle.
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