Barrel Twist

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Gandolf1, Aug 1, 2005.

  1. Gandolf1

    Gandolf1 Well-Known Member

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    Gents

    I post asking for advice /ubbthreads/images/graemlins/smile.gif as you all know alot more about such matters than I.

    I am in the process of ordering my first LR rifle (Cal .308)and have been advised to have a barrel with a 1 in 11 or 12 twist for either a 168g or 180 grain bullet.

    My research indicates that this barrel is only good for stabilizing the bullet if it is travelling at less than 2800fps. Is my research correct?

    If I push the bullet above the 2800fps threshold would I limit barrel life or create throat errosion etc and would it adversely affect accuracy?

    My concerns are round the fact that I think I am being pushed into getting something that may not be truly acceptable and appropriate merely something this rifle manufacture supplies.

    As you will appreciate, this will be the most expensive rifle I will probably buy and currently its my dream /ubbthreads/images/graemlins/smile.gif I don't want it to become an expensive nightmare, any help or advice would be greatly appreciated.

    Thanks

    H
     
  2. nowler

    nowler Well-Known Member

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    a 1 in 12 will stabilise up to 190gr bullets normally, without any worries. i don't know where you are getting your info from, but my understanding of it is that the faster you push the bullet, the more stabilised it becomes, although velocity will not compensate for too slow a twist rate in the first instance.

    anyway, 1 in 11 or 1 in 12 no problems.
     

  3. Gandolf1

    Gandolf1 Well-Known Member

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    Nowler

    According to the Greenhill formulae I have the following data:

    Bullet length 1.139" to 1.237" = 1 in 12 twist for a velocity less than 2800fps.

    or

    Bullet length 1.178" to 1.264" = 1 in 14 twist for a velocity more than 2800fps.

    This is all for a 180g Sierra Gameking round at 1.2055" in length. I accept the bullet is a hunting round but I will be using it for both LR and hunting. So you see the dilema, I'm being told a 1 in 11 or 1 in 12 and the data says a 1 in 14 if I go above 2800fps.
     
  4. Tailgunner

    Tailgunner Active Member

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    Remember that the Greenhill formulia gives you the MINIMUM twist required to stablize the length of bullet being used. Within reason, going with a faster twist is better (1-12 is faster than 1-14)
     
  5. Michael Eichele

    Michael Eichele Well-Known Member

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    Best twists for 308 with various bullets:

    190 1-12 min (1-11 or 11.25 Best)2400-2650

    175, 178, 180 12 works VERY well (2400-2750)

    168 1-12, 1-13, and even 1-14 works well (2500-2900)

    150-155's 1-14 through 1-12 works well depending on weather or not youre using Senars or others. (2600-3000)

    Weather or not you use formulas or real world conditions, you will find that the numbers above will work very well in most conditions.
     
  6. Mikecr

    Mikecr Well-Known Member

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    [ QUOTE ]
    My research indicates that this barrel is only good for stabilizing the bullet if it is travelling at less than 2800fps. Is my research correct?

    [/ QUOTE ]

    Your research is weak so far.
    Do you think a bullet would be less stable ABOVE 2800?
     
  7. Gandolf1

    Gandolf1 Well-Known Member

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    TG & M

    Many thanks for that I appreciate the time you took to respond it has helped me a lot.

    Mikecr

    With respect I suggest you read my post again, as three members have understood my question I make the diagnosis my ability to get across my point is logical and understood. I have clearly made my point that my knowledge is limited so to be told it is weak adds nothing to me trying to learn, but hey! thanks for posting.
     
  8. Michael Eichele

    Michael Eichele Well-Known Member

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    You were getting your idea of below 2800 FPS by the green hill formula. His formulas dictate differant twist needs for differant velocity. 1500-2799 uses 1 constant and 2800 and above uses nother constant in the formulas. The faster you drive a bullet the less twist is needed. Most modern calcs use a gradual increase or decrease instead of 1 FPS being the deciding factor.

    The JBM website has a very usefull twist calculator as well as the RSI and Reloaders Archive computer programs.

    I hope that helps your research.
     
  9. Neverlost1

    Neverlost1 Member

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    Maybe a short description of a lengthy subject may help. You have two factors working against each other when considering barrel twist rates. First is the bullet's imbalance. Most bullets center of gravity and center of spin are not going to be perfect, considering manufacturing tolerences and what happens to a bullet from the forces during firing. The faster you spin a bullet the more it will wobble due to this imbalance, , the worse the dispersion will be, and the bigger your groups will be. For this reason, you would want the slowest twist possible to stabilise you bullet and and keep this effect to a minimum. The second factor to consider is from the bullet's impact with the atmosphere. I believe this has a greater effect on a bullet's stability than the first. This impact with the atmosphere causes the bullet to yaw. To overcome this, you have to spin the bullet faster. As you can see, you have to compromise imbalance stability for atmospheric pressure stability. Will you be using the gun for target shooting or hunting? Will you always hunt in the same area or maybe an elk hunt in the mountains and a deer hunt at sea level? If your going to use the gun at varried locations, you should probably go with the faster twist rate. Here is a link with some outstanding information on the subject:

    http://www.riflebarrels.com/articles/bullets_ballistics.htm
     
  10. Gandolf1

    Gandolf1 Well-Known Member

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    Thanks guys, lots of info to read and take in.

    Elroy, thanks I know understand the topic a little better /ubbthreads/images/graemlins/smile.gif I will be using the rifle as a long range piece and hunting rifle in varied locations etc as you suggested so I guess a faster twist would be better?

    Thanks again all

    H
     
  11. chris matthews

    chris matthews Well-Known Member

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    In your situation, the 1-11 will be perfect. I have hunted from Texas to Alberta with a 308 with a 1-11 and it shoots in the .1s and .2 at 100 and 6" at 1000 no matter where I go if I do my part.
     
  12. Gandolf1

    Gandolf1 Well-Known Member

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    Chris

    Thanks for that, I appreciate your advice the rifle is costing lots of money and its got to be right. As I know you build rifles Chris what would you advise as a spec for a newbie like me in .308?

    H
     
  13. Gerard Schultz

    Gerard Schultz Official LRH Sponsor

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    When dealing with barrel twist and bullet choice, it is better to think in terms of bullet length, rather than bullet weight.

    Try this program for doing the calculations. It will get you closer to the ideal than Greenhill.

    Winload v2.12