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Old December 20, 2012, 07:48 AM   #1
rebs
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AR barrel twist ?

In an AR which twist is best for 55 gr bullets, 1/7, 1/8 or 1/9 ?
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Old December 20, 2012, 07:58 AM   #2
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1/9
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Old December 20, 2012, 10:27 AM   #3
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Honestly I wouldn't worry about it. I've shot a lot of 55 gr bullets through a 1:7 twist.

Most folks shooting 55gr bullets aren't trying for ultimate accuracy, more economical shooting. So most 55gr loads aren't particularly accurate, even out of the 1:12 twist of an A1 barrel.

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Old December 20, 2012, 02:54 PM   #4
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No solid answer as it depends on the gun.

Generally they are all likely to shoot the 55 decently.

1-9 will shoot the lighter bullets better generally but there are lots of exceptions.

1-7 is generally more better for the heavier (62 gr and up) bullets.

And it really only makes a significant difference if you are doing serious target shooting.

All three of them can very likely shoot a 55 gr bullet better than the person doing the shooting.
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Old December 20, 2012, 06:12 PM   #5
Bart B.
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1:12.

That's based on the benchresters favorite twist for 50 to 55 grain bullets fired at the same speed as the .223 Rem. does is 1:14 made popular with the .222 Rem. years ago. With the shorter AR barrel, one needs a slightly faster twist to spin the bullets at the same rpm rate.

Check out http://www.6mmbr.com/223Rem.html

Spinning bullets too fast will shoot those unbalanced ones too far away from group center. The faster a given bullet is spun, the more centrifugal force there is to make it jump off the bore axis as it exits the muzzle. Best accuracy is attained when the bullet is spun just fast enough to keep it stable all the way to the target. 1 grain less powder may not spin it fast enough to do that.
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Old December 20, 2012, 07:01 PM   #6
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55s are good for 1:12s to 1:7s.

I like 1:7s because it gives me the option of going heavier, to 90 grns, but still shoots 55s quite well.
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Old December 20, 2012, 09:18 PM   #7
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Bullet imbalance is a bigger problem with crappy bullets. Sierra started using 4 draw steps for their Match King line to produce very uniform jackets, and good QA/QC on the lead cores produces some of the most uniform bullets for balance on the market.

If you have a mathematically perfect bullet, the twist rate doesn't matter as long as the twist is enough to stabilize the bullet. In reality the difference between a 1:7 twist and a 1:16 twist with a 22 caliber bullet is less than 1/4 MOA at 100.

Standard FMJ bullets aren't exactly the "high quality" specimens you would expect from a match bullet.

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Old December 21, 2012, 01:26 PM   #8
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Quote:
In reality the difference between a 1:7 twist and a 1:16 twist with a 22 caliber bullet is less than 1/4 MOA at 100.
That's not quite true. It may be true for light bullets (52-55) grn bullets in a 1:7 but it certainly isn't true for heavy bullets in the 1:16 (or 1:12)

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