In my experience, any rifles chambered for a .224.......................
bullet using .222; .223; .222Mag; 5.6X50 cases, work better with a 12" twist or quicker.
I think sometimes we get TOO specialized, "Welp, at 20yds, I used the 40Grnr w/th' 8" twist an' th' Clyde & Byed chamber, but th' kicker came when all th' yotees come out th'bar, fallin' all over, an', since it was 2hunnert yds, I quick-changed to the Nizwonger upper an' th' 106's with the cups reamed t' take th' .50(BM) primers, an'...."
Hey, if you're going to shoot a VARMINT(or, should that be, VarMint??) bullet AT a VARMINT, instead of MKs, don't you think you should be thinking of 12" or 10" twists, and 50-64gr bullets, instead of reeling off #s in thousandths?? Remember, its a LIVE target, you are responsible for DROPPPING IT, rather than letting it get away wounded.
One thing about an AR VarMint rifle, the bullet has to fit in the case, and then that case has to fit IN the magazine, unless you're going to use it as a bolt-gun.....
Another thing, in case anyone forgets, the twist on an AR was changed because the longer bullet didn't stabilize properly in the 14" twist in cold weather. "However, shooting on the beach under umbrellas, fellas, we can use....."