My .223/20 Combo .223 barrel does best with Winchester 45 grn HP (Econo Box), 3/4" @ 100 yards (3 shot groups). 55 grn. surplus (Win) opens up to 1 1/2". Anything heavier than 55 grns. will not stablize in the 1 in 12" twist barrel. The new heavy barrel .233 has a 1 in 9" twist to take advantage of heavier bullets.
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