Wow, that went divergent fast! With a 1X9 twist, yes, it will hold accuracy to 600, and even beyond depending on velocity. For a 20" AR you can stretch 69s out to 800 or so. Most 1X9 barrels will also stabilize the 75 and 77 grain bullets as long as velocity is kept up. It was once said by a high power gunsmith a 1X9 twist will stabilize any bullet that can feed through the magazine.
Good post Kurt. The 69 is a great bullet. As for 77's working, I've had issues with those in a 1 in 9 twist. I guess YMMV on that one. The heaviest I shoot in my HBAR is 75gr. It actually prefers 69SMK over anything else I've tried though. The op's question was regarding 600 yards, not 1,000. The 69 definitely holds it's own at that range.
