The 77s won't stableise in a 1:9. Are you target shooting or intending it for game use. 300 yds is in my opinion way past the ethical limit for the 223.
I've been using the 62 grain belmont it's pretty good. When I was reloading for it I tried the 60 grain hornady, pretty unimpressed the 55 gr Sierra hpbt was outstanding for animals out to 200 maybe 220 yards.
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