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GBRaymer
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Posted: 3/20/2007 9:14:04 PM
I have'nt seen much discussion on the subject of barrel twist for the 6.8 SPC. I've noticed one barrel vendor is switching from a 1/10 twist to 1/11 twist. Considering the selection of commercial bullet weights of 110 and 115 grains can there be that big of a difference in performance?
user426
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Posted: 3/21/2007 7:08:04 AM

Originally Posted By GBRaymer:
I have'nt seen much discussion on the subject of barrel twist for the 6.8 SPC. I've noticed one barrel vendor is switching from a 1/10 twist to 1/11 twist. Considering the selection of commercial bullet weights of 110 and 115 grains can there be that big of a difference in performance?



Do a search....that's a hot topic here....
mr_wilson
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Posted: 3/21/2007 7:18:38 AM
Welcome aboard ,

Sorry but the guy above does not realize you are very limited on your search options here as you are not yet a team member.

Here are a couple threads discussing 6.8mm barrels and twists:

www.ar15.com/forums/topic.html?b=3&f=121&t=315437

www.ar15.com/forums/topic.html?b=3&f=121&t=318030

May get you started, please note that at the top of this forum there is boo-coo 6.8mm rifle info which you have probably already noticed, it's a good place to start.

FWIW, I have a CMMG 6.8mm 16" rifle and it prefers the Silver State 115grn SMKs.

Hope this helps,
mike
Life Member NRA, TSRA, LEAA, LSBA
Certified NRA Instructor: Pistol & Personal Protection
"Light travels faster than sound. That's why some people appear to be bright until they say something.ā€¯
Ko-tonics
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Posted: 3/21/2007 11:19:09 AM
[Last Edit: 3/21/2007 11:30:16 AM by Ko-tonics]

Originally Posted By GBRaymer:
I haven't seen much discussion on the subject of barrel twist for the 6.8 SPC. I've noticed one barrel vendor is switching from a 1/10 twist to 1/11 twist. Considering the selection of commercial bullet weights of 110 and 115 grains can there be that big of a difference in performance?

Cliff notes version:

- Rifling twist rates that are faster than necessary to stabilize particular size bullets reduce velocity. Match rifle shooters have known for decades that you want the LEAST twist rate which stabilizes the bullets you are shooting. Friction, wear and crud are reduced with slower twist rates. At the hairy fringe of accuracy, there is some evidence that twist rates that are too fast for a particular bullet can actually reduce accuracy at long ranges.

- When used in an AR-15, the size of the magazine dictates that shorter bullets will be more often used. This is because longer bullets cannot be seated-out in the case, and if they are pushed down in the case so that the maximum overall length doesn't exceed ~2.26", then the bullet is taking up room in the case, which will reduce available volume for powder, resulting in lower velocities. The longer the bullet, the faster the required twist rate. Shorter bullets don't need fast twist rates.

- The Greenhill formula for calculating twist rates is conservative, meaning that it tends to dictate a faster twist rate than necessary. Nonetheless, typical 6.8/AR-15 bullets in the 90 to 115 grain range will be stabilized from 1:11.5" to 1:13" per Greenhill. The only exception to this is the long 110 grain Barnes Triple-Shock X bullet, which Greenhill says needs a 1:10.5" rate. However, Barnes engineers have told me that even that particular bullet can be stabilized with a twist rate as slow as 1:12.

- The SAAMI specification for 6.8 SPC recommends 1:10", but this is listed as optional, and SAAMI does not differentiate between bolt guns and AR-15's. So there's nothing "wrong" from a SAAMI perspective with 1:11", especially when considering the particular rifle we know and love.

Summary: For 6.8 SPC, 1:11" twist is much better than 1:10" for the AR-15, and is still on the conservative side. You need not worry that your bullets will be understabilized. And your barrel will get better velocity, be easier to clean, wear longer, and maybe even be more accurate.

Ko-tonics
Put your AR-15 on steroids: 6.8mm SPC
kalwasart
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Posted: 3/21/2007 2:03:27 PM
Tim is correct on all points and to add to that, with a slower twist you can go to a heavier bullet and have less concerns about pressures and get better velocites over the 1 in 10


Art - SSA
arteacher
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Posted: 3/21/2007 6:33:23 PM



This is a 5 shot group at 100 yards with a 1/9.5 Kotonics barrel.

How much more accurate can 1/11 be?
DaveS
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Posted: 3/21/2007 8:34:07 PM
[Last Edit: 3/21/2007 8:38:26 PM by DaveS]

Originally Posted By arteacher:
i176.photobucket.com/albums/w189/ar-15teacher/100_0532-1.jpg


This is a 5 shot group at 100 yards with a 1/9.5 Kotonics barrel.

How much more accurate can 1/11 be?


About another 67% tighter with me behind it.

BTW, you did real good with that Ko-tonics upper. Not too shabby at all.

I'd put money down on ya'.
Atgr
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Posted: 3/21/2007 9:08:38 PM
ARTEACHER

What load are you using? hy.gif
arteacher
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Posted: 3/21/2007 9:31:17 PM

Originally Posted By Atgr:
ARTEACHER

What load are you using?


Silver State brass, Rem 71/2 benchrest primer 28.5 grains H322 Sierra Prohunter 110gr
Ko-tonics
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Posted: 3/22/2007 9:59:20 AM

Originally Posted By arteacher:


This is a 5 shot group at 100 yards with a 1/9.5 Kotonics barrel.

How much more accurate can 1/11 be?


That's a good question, Mike. What I read was that if a bullet is spinning faster than necessary to stabilize it, it does not pitch forward correctly at long ranges. Apparently, if a bullet is spinning at the right speed, the pressure of the air as it begins to fall makes the point pitch slightly downward so that it follows the trajectory. If the bullet is spinning too fast, it doesn't pitch down, so that the airflow over it's sides is disrupted, and accuracy is affected.

None of this has been proven or seen by me. I am lucky if I can shoot a 1-inch group at 50 yards. But the point is that the right spin rate gives you better accuracy at long ranges. I believe we are talking about >450 yards.

Ko-tonics
Put your AR-15 on steroids: 6.8mm SPC
southpaw67
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Posted: 3/22/2007 11:59:44 AM
Originally Posted By arteacher:
i176.photobucket.com/albums/w189/ar-15teacher/100_0532-1.jpg


This is a 5 shot group at 100 yards with a 1/9.5 Kotonics barrel.

<snip>quote]


Nice shootin Tex! What optic were/are you using? I read your answer to what load this was.
"Without a sign, his sword the brave man draws, and asks no omen, but his country's cause."......Homer
arteacher
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Posted: 3/22/2007 5:18:38 PM

Originally Posted By southpaw67:
Originally Posted By arteacher:
i176.photobucket.com/albums/w189/ar-15teacher/100_0532-1.jpg


This is a 5 shot group at 100 yards with a 1/9.5 Kotonics barrel.

<snip>quote]


Nice shootin Tex! What optic were/are you using? I read your answer to what load this was.


The 6.8 has a Simmons Aetec 3x12 with the lit crosshairs. Most all of my pig hunting is at night and my old eyes can't see the crosshairs on the pig otherwise.