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Old September 26, 2013, 01:17 PM   #1
Drhc116
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Is 1 in 7 twist too fast for 20" AR??

So I just bought a PSA CHF 20" upper with 1 in 7 twist. While I was speaking with a few of my buddies yesterday, one of them mentioned that he thought that the 1 in 7 was a little fast for that long of a barrel. I have read multiple threads and the opinions seem to go both ways. I plan on shooting mostly 62gr LC surplus with the occasional case of 55gr commercial thrown in. Should I ditch the FN cold hammer forged 1 in 7 and go to something else or do you all think it will serve my paper punching duties fine like it is? I also think I may handload some of the SMK in the heavier weights for coyotes as well.

Thanks.
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Old September 26, 2013, 01:42 PM   #2
jmr40
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Barrel length has nothing to do with it. 1:7 generally works best with 60gr and up. It should do fine with 55 gr as well. Probably not a good choice for some of the 40 gr ammo.
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Old September 26, 2013, 02:11 PM   #3
kraigwy
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The 1:7 is perfect for an AR in my opinion. it will shoot anything from 50-90 gr bullets just fine.

The Army uses the Mann Accuracy Device to test their ammo. They use 52 Gr SMK loads as a control. The Mann Device is a 1:7.

Only problem you'll have is using light 40 gr bullets for the Hornet and Hornet velocities.
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Old September 26, 2013, 02:50 PM   #4
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I think it is MORE appropriate for 20" than 16", though the barrel length has nothing to do with effect of twist rate. The reason I say that is that the 20" is more suitable for longer range work and heavier/longer bullets are better for longer ranges. Service rifle competitions shoot out to 600 yds and the 75 gr - 90 gr bullets are great for that. And for longer range having a longer barrel the 20" will give slightly more velocity.

I have a PSA upper with 1:7" twist and it shoots everything from 55 gr to 77 gr very well (1/2" - 7/8" 5-shot groups are easy with handloads).
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Old September 26, 2013, 04:35 PM   #5
Bartholomew Roberts
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All of my 5.56 rifles are 1:7 or 1:8. I find it to be more versatile and useful. I have never had an issue with it being too fast - even with WWB 45gr JHP.
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Old September 26, 2013, 05:12 PM   #6
Drhc116
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Thanks for the replies everyone.

To be honest I really do not think that I will ever shoot anything under 55gr unless that is all I can find. If that is in fact the case, I would simply shoot it out of one of my 1 in 9 carbines. I have 2 carbines currently and have always wanted a rifle length upper so i decided to take the plunge on a complete upper with BCG and CH. If i shoot anything other than 55gr or 62gr it would be the heavier weights.

Once again thanks everyone.
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Old September 26, 2013, 06:27 PM   #7
CharlieDeltaJuliet
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I agree that the 1-7" is a great twist for larger projectiles. I have a RRA Varmint 24" barrel AR that has a 1-8" twist. I have yet to use anything that it does not shoot good, but I am a sucker for a fast twist barrel.
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Old September 26, 2013, 06:41 PM   #8
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I don't want to hijack the thread, but I believe it's in the same vein as this thread. Is the only problem with a fast twist with lightly constructed bullets of lighter weight? Can you really overspin a bullet in regards to accuracy?
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Old September 26, 2013, 06:46 PM   #9
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I don't think it will fit your needs please send me your information so i can get a UPS pick up ticket to you to send it to me. Just kidding. I love the 1/7 twist for everything i shoot. Enjoy.
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Old September 27, 2013, 12:29 AM   #10
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Quote:
Originally Posted by jmr40
Barrel length has nothing to do with it.
Quote:
Originally Posted by NWPilgrim
the barrel length has nothing to do with effect of twist rate.
But it does. A shorter barrel produces lower velocity. And to achieve the same level of stability, a slower bullet needs to be spun a little bit faster than the same bullet at a higher velocity.
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Old September 27, 2013, 04:55 AM   #11
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1/7 twist is fine for a 20" barrel, you can stabilize the heaviest bullets in .224 with it. I have heard stories about really lightweight bullets shedding the jacket at these high RPMs, but I shoot 55 grain bullets out of mine @ 3200 fps all the time without problem, mathematically this works out to well over 300,000 rpm (at the muzzle).
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Old September 27, 2013, 05:55 AM   #12
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Is 1 in 7 twist too fast for 20" AR??

Quote:
Originally Posted by Theohazard View Post
But it does. A shorter barrel produces lower velocity. And to achieve the same level of stability, a slower bullet needs to be spun a little bit faster than the same bullet at a higher velocity.
You are correct that indirectly the barrel length has an influence on bullet RPM by way of greater velocity from a longer barrel (to a point). The difference in velocity between same load in 16" and 20" bbl is at most 200fps compared to say 3,000 fps total or about 7%. Which in practical terms will have no noticeable effect on bullet stability in the normal range of of bullet wright lengths used for 1:7 twist rifles. Perhaps at the margin such as 45 gr bullet the 20" would be slightly better. But I doubt there is any common circumstance in which the 20" bbl is significantly WORSE as implied by the OP's brother.
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Old September 27, 2013, 09:32 AM   #13
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Quote:
Originally Posted by NWPilgrim
But I doubt there is any common circumstance in which the 20" bbl is significantly WORSE as implied by the OP's brother.
You're right, it's a pretty small difference and probably not really noticeable in this case. But my only point was that barrel length does affect a round's ideal rifling twist, even if only by a small amount.
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Old September 28, 2013, 11:44 PM   #14
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Quote:
Can you really overspin a bullet in regards to accuracy?
Yes. The faster the spin, the more that imperfections in the bullet construction will affect accuracy. Ideally you want to spin it just fast enough to keep it stable, and not too much more.

How big of an effect on accuracy? Probably not enough to matter for most people with most rifles and most ammo. If you are nursing every little bit of accuracy out of your system, then it becomes important.

-J.
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Old September 29, 2013, 12:36 AM   #15
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1:9 is fast enough to stabilize anything that is short enough to fit in an AR magazine. With a barrel under 20" and beyond 500yds you could run into trouble stabilizing bullets 75gr or heavier. Other than that, there's no need to go faster than 1:9.

Here's some information on .223/5.56 Twist rate.

http://thefiringline.com/forums/showthread.php?t=530927
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Old September 29, 2013, 01:41 PM   #16
Bartholomew Roberts
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I've actually run into problems with a 16" 1:9 Bushmaster HBAR stabilizing 75gr and 77gr ammo at 100yds. I was getting 4-5" groups with match ammo. No keyholing but really poor accuracy. At 200yds, I couldn't even keep them on the paper to figure out the group size.

However, I've watched a friend's 16" Armalite 1:9 shoot the same 75gr ammo into a 1" 5-shot group at 100yds.

For me, my general rule is if I know I want to shoot 75gr and up, then I buy a 1:8 or faster twist.
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Old September 29, 2013, 03:02 PM   #17
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Quote:
Originally Posted by JohnKSa
1:9 is fast enough to stabilize anything that is short enough to fit in an AR magazine. With a barrel under 20" and beyond 500yds you could run into trouble stabilizing bullets 75gr or heavier. Other than that, there's no need to go faster than 1:9.
While that may be true down south, it doesn't work that way up here when it gets cold. 1:9 works fine in the summer, but when the temp gets close to zero, 1:9 from a 26" Savage 12FV will keyhole a 75gr HPBT at 50 yards, while 69gr is still accurate. My 1:8 twist AR still stabilizes both of them in the cold.
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Old September 29, 2013, 07:27 PM   #18
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Very interesting information. I'll amend the other thread to reflect those experiences.
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Old September 29, 2013, 09:07 PM   #19
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One of the factors in the Miller stability formula is air density, and temp and altitude are both a big factor in that.

Also, once a bullet is stable, it will stay that way, even when it gets pushed around as it drops trans-sonic. While forward velocity drops, rotational velocity doesn't, and it works out that the stability factor actually increases as it goes downrange.

Here's something to play with:

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

If you really want to get into it, read this:

http://www.jbmballistics.com/ballist...tability_1.pdf
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Old October 1, 2013, 08:52 PM   #20
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I had my 1 in 8 twist 20 inch NM barrel rip apart some no name brand 50 grain hp bullets, but otherwise it works fine. I would stay with the heavier bullets for better accuracy.
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Old October 5, 2013, 03:35 AM   #21
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I have the 1 to 7 in my Sig rifles and I find they are more accurate with the heavier 62 to 75 grain bullets
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Old October 5, 2013, 02:37 PM   #22
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I just sold my HK SL8. It has a 20" barrel (510mm) and a 1in7" twist (178mm).
In the manual HK even recommends using 52gr Sierra MatchKings.
Therefore I'm a) sure that a 20" AR15 with a 1in7" twist is not a bad choice and b) that a rather long barrel with a rather short twist will be a good for a wide range of bullet weights, at least from 52 to over 75gr.
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Old October 6, 2013, 04:43 AM   #23
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I shoot Service Rifle with a 1:7 Colt barrel. It shoots 75gr and 80gr bullets very well. I've used 55gr FMJ bullets for the 200 yard standing stage, and that worked just fine from an accuracy standpoint. I had them to use, so I used them up. Switched to heavier bullets for the 300 and 500 yard stages.

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Old October 8, 2013, 09:47 PM   #24
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Unless you plan on shooting 77 gr consistently yes 1/7 is too fast a twist. 1/8 is a better choice all around.
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