I'd have a hard time believing it won't stabilize in an 8. BUT the 8 would have to be a true 8 or faster. And the bullet probably can't be a slow load. Just somewhat educated guesses there.
Edit to ad, just saw the 16 inch tube, that one its possible you have all working against you. Barrel twist 8 plus heading to 9. Slow load. Short barrel, equals low MV and not what you need when you are on the edge.
That said your bullets should have been oblong holes instead of round if that was the case and I"ve shot some pretty decent 300 yard groups with oblong holes...
I've seriously been noodling on whether or not to ladder test down from the max load stated in the 5.56 section of Hornady 11th. Say 3 half grain steps. That might relieve my curiosity. I have 172 of the damn things. What's nine more?
