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Centerfire Rifles - Semiautomatic or Gas Operated Centerfire rifles, carbines and other gas operated rifles.

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  #1  
Old 01-07-2010, 8:03 AM
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Default .308 Twist Rate

What bullet weights would a 1:12 twist rate be appropriate for in a 22" barrelled .308?
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  #2  
Old 01-07-2010, 8:17 AM
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the easy answer is to say something like 175gr. i shoot 175 SMK from my 24" barrel (1:11.25 twist) and they shoot just fine and dandy. a buddy of mine shoots 190gr SMK from his 700 (24" 1:12 twist) and they shoot pretty darn accrit.

the best thing to do is to shoot some different weights at the distances you plan to be shooting at. that'll tell you what your rifle likes best.

Last edited by swerv512; 01-07-2010 at 1:17 PM..
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Old 01-07-2010, 10:51 AM
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I shoot 125's through 175's in my 1:12" M1A Supermatch in competition.
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Old 01-07-2010, 11:57 AM
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I shoot 168gr OTM through my 1:12 20" barrel with great results.
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Old 01-07-2010, 12:18 PM
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The 1:12 twist rate will work best with bullets up through the 175 gr. Sierra Match King (IIRC it was designed for the 1:12 twist military rifles). Once you start getting heavier than that (180 and up), you'll do better to have a faster 1:10 (or 1:11) twist.

That is not to say that a 1:12 won't shoot 180's and heavier worth a darn. The average guy with a stock hunting rifle will be hard pressed to notice a difference, but the High Power/Long range competitors probably will.

For the average big game hunter shooting 150 gr - 180 gr bullets, I think you'd be hard pressed to notice a difference between a 1:10 vs 1:12 barrelled rifle.

Last edited by plsmyk; 01-07-2010 at 12:21 PM..
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Old 01-07-2010, 1:44 PM
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I've heard of people using 208 A-Maxs with sucess.
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Old 01-07-2010, 2:37 PM
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Depends on Altitude.
At sea level, you may not want to go heavier than 168gr bullets.
At higher elevations, I would stop at 175gr SMK's.
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Old 01-07-2010, 2:42 PM
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If you have Excel, enter this formula (Miller's stability formula) into cell B1:

=(30*B3)/(((B4/B5)^2*B5^3*(B6/B5)*(1+(B6/B5)^2)))

cells B3 to B6 covers these values:

B3 = bullet mass in grains
B4 = twist rate in inches
B5 = bullet diameter in inches
B6 = bullet length in inches

You're looking for an output value of 1.40 or greater.
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  #9  
Old 01-07-2010, 2:43 PM
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I shoot 150gr-180 gr in 1-12 twist barrel chambered for 300 winmag. I have no problems.
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Old 01-07-2010, 2:45 PM
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Quote:
Originally Posted by ar15barrels View Post
Depends on Altitude.
oops, forgot those two other equations...

inc...
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Old 01-07-2010, 2:50 PM
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Quote:
Originally Posted by HUTCH 7.62 View Post
I shoot 150gr-180 gr in 1-12 twist barrel chambered for 300 winmag. I have no problems.
i'm sure those 300WM velocities have something to do with it...
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Old 01-07-2010, 2:54 PM
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Quote:
Originally Posted by God Bless The Mauser View Post
I've heard of people using 208 A-Maxs with sucess.
Took the words right out of my mouth. Was that on the hide? I think the guy was trying to see if they would destabilize but he had no luck. Still worked.


EDIT: I wonder what altitude the guy was at? Hmm
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Old 01-07-2010, 2:57 PM
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Quote:
Originally Posted by Jpach View Post
Took the words right out of my mouth. Was that on the hide? I think the guy was trying to see if they would destabilize but he had no luck. Still worked.
Factory barrel twist rates can generally run +/- 1" from the stated twist rate and still be called within spec.
So, it's quite possible to have a 1:12 marked barrel that's really 1:11 twist.
We can not expect this to be the case with another barrel though so it's not safe to assume that something that worked in ONE person's gun will work any anyone else's gun.
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Old 01-07-2010, 2:59 PM
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velocity correction factor:

=(B10/2800)^(1/3)

where B10 is bullet speed in fps

atmospheric correction:

=(B14+460)/(59+460)*29.92/B15

where B14 is temperature in Farenheit, B15 is atmospheric pressure in inches of mercury

multiply all 3 outputs to get the final Sg...
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Old 01-07-2010, 3:15 PM
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Quote:
Originally Posted by Jpach View Post
Took the words right out of my mouth. Was that on the hide? I think the guy was trying to see if they would destabilize but he had no luck. Still worked.


EDIT: I wonder what altitude the guy was at? Hmm
That's where I saw it. I'm thinking about trying some of those bullets. The down side is I probably won't be able to load to mag length for my SPS-V http://www.snipershide.com/forum/ubb...1422055&page=1
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  #16  
Old 01-08-2010, 8:21 AM
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Thanks for the input. Every time I think I know something about a subject, I find out there are always people out there who know a lot more:-)
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