Frank, just read the rest of this thread. In your 6.8 pistol, when is the last time you cleaned it? Look in the end of the barrel and see if there is copper buildup (copper fouling), this can throw accuracy all over the place.
Next is your gas. Check that the gas block had not come loose, a change in gas every shot will change POI dramatically. Also pull out your bolt carrier and check that the gas key is still tight.
Shouldn't have anything to do with the barrel nut because the gas tube holds it in place somewhat, unless it was not properly aligned/torqued in the first place.
Ditch the cheap scope and mount. Don't want to sound like a dick, but why own a nice gun and put a **** scope and mount on it. Something that works on a .22 may not work in a 6.8. Also, the scope can go bad. So if that scope no longer works in the .22 and even if it does, may still not work on the 6.8 or others. Get a good mount. There are ones made for the AR that are cheap, Burris PEPR, AR Stoner, Aero Precision, etc.