We shoot tons of 80 class bullets in the slow fire prone event of the CMP and NRA matches. By rule matches are single load only. Our barrels range from 6.5 to 8 twist. Some 8s will shoot the 80 class and we found just would not shoot them precisely at 600 yards no matter how much effort is undetaken.
Here ia the issue, some button barrels the twist tolerance is the culprit. You may emd up with the slow side of the tolerance 8.xT, that would not shoot the 80s. And if you are lucky you may end up with a barrel faster than 8.0T.
When we say barrel won't shoot, it means low X count at 600, or shots not on call. One of our top shooters, the President's Hundred overall winner at Camp Perry in 2018, called and told me about his barrel that will shoot 200 at 600, but low single digit X count. He rebarreled, the next match he shot 200-13, all shots on call.
Manufacturing tolerance might get you.
Another shooter, an accomplished Palma shooter who dabbles in service rifle competition had an 8T barrel that just will not shoot any 80 class bullets. He tried prone with sling on the bench, off a bipod, and finally a machine rest. Still would not shoot any 80s, any load, any powder, any primer. Off the competition AR it went. It shot the short range (200-300) ammo loaded with 77 class bullets with no issues.