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The AR15.com Ammo Oracle
My Sig556 is 1:7 and shoots 223 55gr perfectly well/accurately at 50-100 yards. Have not tried it further.
Opinions (Pro and Con):
1:9 is best.
Why? Flexibility. It doesn't seem to have any problems throwing M856 tracers around, unless it gets really cold, it wears better than 1:7 and it stabilizes more rounds than 1:12. Additionally, 1:9 rifles, even Mil-Spec chrome chambered and barreled, can attain 1.0-2.0 MOA out to 300+ meters.
No, 1:7 and 1:8 are the best.
Why? Accuracy. For heavier and longer rounds during competition shooting, 1:8 and 1:7 twists are the best for heavy 77-80 grain rounds that I use to shoot competitively at 500-1000 meters. Who needs to shoot tracers anyhow? More importantly, heavier rounds are showing very good results in terminal testing and are proving to be much better defensive rounds.