Quoted:
Does twist rate affect bullet yaw/tumble and terminal effectiveness. This is the question I was requesting information on, Im sorry, I should have made that clearer.
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PD, I 'think' what you're really after is "terminal effectiveness".
If you're determined to use a 14.5" barrel, and that's what you're asking about, go to a 1:7 twist barrel and move up to 75-77grn bullets. The heavier bullets will give you the knock-down power and penetration that I believe you're after - within about 100 yards. The military had to make the move to heavier bullets based on their experience in the Afghan.
Please remember that the AR-15 is an SCHV weapon. Without velocity (or heavy bullets) you've just got a .22. If I were going to spec a weapon for what I think you're after, I would use an 18" barrel with A 1:9 twist and 69grn bullets. But that's just my opinion.
(I'm too tired to get in a long pissing contest tonight.)
5SUB