I have 2, both 16 inch carbine M4 clones, not "rigs" or anything else. Yes, they are the more correct chamber with a 1:10. Xman covers the 1:10 and why it is actually good. They shoot better than me. When I bought the first, the ad was 1 MOA, now it is 1.5, but both of mine are better than that.
I have 2 Rock River 6.8's. They both shoot 1/2 MOA with handloads (when I do my part). Solid rifle. I would jump on it.
I have two RRA uppers in 6.8 and love them both. Since leaving NY for GA I've harvested all my deer with the 16" Coyote Carbine (except one via muzzleloader), it's my go-to hunting rig. I also have the heavy barrel 20" varmint version, basically for paper punching. Both will shoot 1/2 MOA when I do my part.