The 1:12 twist will not effectively stabilize rounds above 62 grains. The 1:8 55-69 very well. But I would recommend staying with the 55 grain bullet since you have two different twist rates. I have a Rock River A-4 Varmint in 1:8 twist and on a good day with P223E Federal Ammunition in 55 gr. will group 1/4 inch groups on occasion. Doesn't get any better than that. So you can very effectively use the 55 gr. round in both. But a 69 grain round will keyhole on the target shot from a 1:12 twist.
Velocity X *12=__________ Divided by the Twist Rate (12) (9) (8) or (7) =____________X *60 = RPM of the bullet leaving the bore! Equals RPM. (Choose Your Twist)
We know most all 223/5.56 Ammunition velocity is more than 3000 FPS but we will use that velocity in the below examples.
This gives you an idea of the Revolutions Per Minute it takes to stabilize a in flight.
For example if the velocity was 3000 FPS the 1:12 Twist would equal 180,000 RPM while the 1:8 would equal 270,000 RPM. Using the formula above. A general rule is the heavier the grain bullet the faster the twist rate must be to stabilize the round in flight. The numbers with an * above are consonants in the formula always representing *Inches (12) and *Minutes (60).
This does not mean that a 55 will not shoot good in a 1:7 but only that peak accuracy will normally be with heavier grain bullets.
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Last edited by Sniper03; 09-17-2013 at 02:46 PM.
Reason: Adding Picture