I have two AR-15 .223 Rem, a Shilen and a Bear Creek, 24 inch 9 twist , when I want to shoot light bullets. I save the barrel burners for long bullets at extreme distance and windy conditions. Just out of curiosity, describe your 500 or 600 yard 5 shot groups with fifties?My 22-250 AI is a 1:7 twist and I have a super fast and accurate load for it, bit with 50 grain bullets. If you are not married to your heavier bullet I can share my load with you. PM me if you have interest.