My M77 with a 1:12 will stabilize 62 grain out to 500 yards and beyond, but that is with a 24" barrel.sunray is correct. Twist 1:12 will stabilize up to and including 55gr bullets; 1:9 good for up to and including 69gr; for bullets heavier (longer) than that you need 1:8 or 1:7 twist.
AR15's are produced with some very short (sub-16") barrels and they are a perfectly adequate rifle to 300M