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5.56/.223 Bullet Weights in 1:9 twist barrels

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Old 10-23-2008, 04:34 PM   #1
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Default 5.56/.223 Bullet Weights in 1:9 twist barrels

Hello all:

I have an older (1980's) vintage COlt SP1 AR-15, and am also waiting on delivery of a new RRA AR. My older Colt has the 20" barrel with (I believe) a 1:12 twist. The new RRA will have a 16" barrel with a 1:9 twist.

I am about to start re-loading again after a long hiatus, and started re-evaluating bullet weights. All of my current .223 ammo inventory is 55 grain FMJ, mostly my old re-loads with just under 2,900 FPS muzzle velocity. I have read a few cautions about light weight (55 grain) bullets not working well in rifles with "fast" twist rates of 1:6.

What is the collective experience out there with using 55 grain FMJ bullets in AR's with a 1:9 twist. Do these bullets perform well, or do I need to start loading the heavier 62 grain bullets?

Thanks in advance,


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Old 10-24-2008, 01:53 AM   #2
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As general rule a faster twist is needed to stabilize a heavy bullet and for no better choice of words the term I have heard used is that a faster twist "over stabilizes" the lighter bullet making it less accurate. That being said I am not sure how one can over stabilize anything. I think you would be happier with your existing 1 in 12 twist and 55 grain bullets, I know I am, unless you plan on long range target shooting. The heavier bullet (longer) will have a better ballistic coefficient over the lighter one but will not go as fast. I suspect that the military changed to the heavier bullet because of sectional density for better penetration and perhaps more humane wounds. I have not read a word to that effect I am just guessing.

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Old 10-24-2008, 04:26 AM   #3
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55gr work well in a 1:9 twist barrel -- probably plenty accurate, unless you're looking at match grade.

(For that matter, I've had good experiences with 62gr SS109 in a 1:9.)
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Old 10-24-2008, 05:35 AM   #4
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I would like to add one comment about the twist issue that most people don't think of and that is a rifle barrel with a 1 in 12 twist causes a bullet traveling 3000 fps to spin at the incredible speed of 180,000 rpm and a 1 in 9 at 240,000 rpm. If you were to increase that velocity to lets say 3300 fps your 1 in 9 twist is going to be spinning that bullet 10% faster or 264,000 rpm which is in the area that a lead core bullet will come apart due to centrifugal force. My point being that if you were to decide to shoot any bullet that you could drive somewhere in that 3300 fps range with a 1 in 9 twist it will not only not be accurate, and depending if it where a lead core bullet, you could find that the bullet will never reach the target because it vaporized before it ever got there.
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Old 10-24-2008, 06:27 AM   #5
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55gr bullets do very well in 1/9 twist barrels.
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Old 10-31-2008, 01:21 AM   #6
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X2, 55 does very well in a 1:9

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