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#6953035 - 11/09/17 11:03 AM 1:'9 1:9.5 or 1:10 twist for 7mm-08?
tenyearsgone Offline
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Registered: 04/21/13
Posts: 1617
I'm getting back into this project, and need a barrel for my AR-308. It seems like splitting hairs with these twists though. I would like to use 140 grain bullets, and will reload.

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#6953046 - 11/09/17 11:11 AM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
kmon1 Online   content
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Registered: 09/27/06
Posts: 23300
Loc: Texas
First rifles Remington chambered in it were for 140gr bullets and had a 1:9.25 twist, have one in the safe. What barrel length are you planning on running?

If buying a barrel for one today I would go 1:9 or what ever the Berger twist rate calculator shows would run 162gr bullets. Just making sure to have other options in bullet weight.
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#6953110 - 11/09/17 11:52 AM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
FiremanJG Online   content
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Registered: 12/16/08
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Loc: Wolfe City, TX
1:9 7mm-08 will run 140's up to 168's.
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#6953172 - 11/09/17 12:36 PM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
dee Online   content
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Registered: 04/10/10
Posts: 5372
Loc: Red River way
9 twist will do it all. Depending on speed and bullet choice one could get by with a 10 but it's not ideal imo.
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#6953345 - 11/09/17 03:05 PM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
Dan S. Offline
Light Foot

Registered: 09/08/16
Posts: 14
I'm 9.5 on ours and they handle everything we shoot

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#6953780 - 11/09/17 09:29 PM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
ChadTRG42 Online   happy
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Registered: 09/16/09
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Since an AR already loses 100 fps compared to a bolt gun, I'd go faster twist than what you need. Most 140 grain will work fine in a 1:10" twist. But it wouldn't work well for the 162-168 grain bullets as good. It would handicap the rifle some. I would lean heavily to a 1:9" twist. Plus the faster twist helps with better terminal performance.

I went 1:11" twist on my 300 WM since it was "just enough". I now wish I had gone with a 1:10" to fully stabilize the heavy 30 cal bullets.
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#6953795 - 11/09/17 09:48 PM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
jeffbird Online   content


Registered: 03/09/09
Posts: 3113
Chad,

why do you say a 1:9” will have better terminal performance than 1:10”?
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#6953865 - 11/10/17 12:32 AM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
kmon1 Online   content
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Registered: 09/27/06
Posts: 23300
Loc: Texas
I think Chad is referring to RPS on the bullet, the faster it is spinning the more centrifugal force is exerted making it open a little faster giving a larger frontal diameter a little quicker and separating out those secondary projectiles John Nosler loved for secondary wound cavities. With barnes bullets it helps initiate expansion a little quicker.
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#6954027 - 11/10/17 07:40 AM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
jeffbird Online   content


Registered: 03/09/09
Posts: 3113
Just to make math easy, say a deer is 18” thick.

1:9” will make two cimplete revs passing through. 1:10” will make 1.8 revs.

Does not seem like a difference of significance assuming all other variables are the same.
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#6954084 - 11/10/17 08:24 AM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
ccoker Online   content
Extreme Tracker

Registered: 09/17/09
Posts: 3912
I went through a discussion here recently on the my recent 7mm08 project with a 1:10 twist.

Results: 17" barrel is shooting 140s and 120s very well.
140 averages 2800 FPS
120 is 2900 FPS

I am digging this little bolt action, too it out yesterday evening and center punched a doe through the top of the head from a tower blind with the 139 GMX.

BTW, I have quite a bit of factory ammo available I will sell off if you want to test some.


Edited by ccoker (11/10/17 08:25 AM)
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#6954087 - 11/10/17 08:25 AM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: jeffbird]
ChadTRG42 Online   happy
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Registered: 09/16/09
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Originally Posted By: jeffbird
Chad,

why do you say a 1:9” will have better terminal performance than 1:10”?


Hey Jeff, basically, what kmon explained. When a bullet is spun faster (faster twist), the bullet has more rotational force. When the bullet enters soft tissue and the bullet is upset (expands), it will have more expansion and more of a chance of tumbling and causing more damage. It's the same principal why the military went to a 1:7" twist in the M4's, from the previously slower rifle twists of 1:9" and 1:12" twists. Since NATO regulates the type of bullets the military can use (FMJ, etc), the bullets are difficult to expand. So when you spin them faster with more rotational force, an FMJ bullet will tumble in soft tissue and cause massive damage. It kind of does the same thing with hunting bullets, and allows for the bullet to expand and possibly fragment more.

For example, I had a customer shoot a large buck in the neck last weekend with a 1:8" twist 28 Nosler and 195 grain EOL bullet from about 140 yards. The bullet went in, and never came out. The rotational force on this bullet at that close of range caused massive fragmentation.
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#6954108 - 11/10/17 08:36 AM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
jeffbird Online   content


Registered: 03/09/09
Posts: 3113
Chad,

the difference is 2 revs (or less) vs. 1.8 (or less) revs passing through the deer.

Hard to see how that 0.2 extra rev could be a difference of significance.
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#6954128 - 11/10/17 08:46 AM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: jeffbird]
FiremanJG Online   content
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Registered: 12/16/08
Posts: 25216
Loc: Wolfe City, TX
Jeff,

You're leaving out the revolutions the bulley made on the way there.
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#6954147 - 11/10/17 08:55 AM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
jeffbird Online   content


Registered: 03/09/09
Posts: 3113
Not following you? Please explain.

The bullet makes one rev in 9" of linear travel or 10" of linear travel.

If a deer is 18" thick (most are less, just picking that to keep math simple), with 1:9" twist, the bullet makes one complete rev over 9" of distance. If the deer is 18" thick, the bullet will make two complete revolutions passing through. With 1:10", the bullet will make 1.8 revolutions passing through.
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#6954256 - 11/10/17 09:44 AM Re: 1:'9 1:9.5 or 1:10 twist for 7mm-08? [Re: tenyearsgone]
P_102 Offline
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Registered: 09/23/08
Posts: 1115
Loc: Grapevine
Jeff, it's not the number of revs, it's the bullet expansion that results from the faster revs. Don't think about 1.8 vs. 2 revs, think about the bullet expanding more and causing more damage. P_102
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