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twist vs velocity

Discussion in 'Rifle Country' started by xtarheel, Apr 26, 2012.

  1. xtarheel

    xtarheel Well-Known Member

    Just wondering.. I have two AR's. One has a 1 in 12 twist the orher 1 in 8.

    I understand the reason for the faster twist is to stabilize heaver bullets. But all things being equal, barrel length, etc. would the rate of twist have any effect on velosity?
  2. Robert

    Robert Moderator

    I could be wrong but velocity is a function of powder burn rate and barrel length. All the twist does is stabilize the bullet, it has no effect on velocity that I know of. But again I could be wrong.
  3. chrome_austex

    chrome_austex Well-Known Member

    Bullets are pretty easy to spin down their long axis, so a faster twist won't eat much energy.

    But ya, there is some loss there. It just may be less than the difference between two same-twist barrels from different manufacturers.
  4. MCMXI

    MCMXI Well-Known Member

    I'd agree with this and add that any differences between the 1:8 and 1:12 barrels would be masked within the normal velocity variations for a given load in the same barrel.
  5. TonyAngel

    TonyAngel Well-Known Member

    It's really nothing to expend any brain cells on.
  6. Float Pilot

    Float Pilot Well-Known Member

    The twist rate accommodates the bullet's length and not the weight.

    A tighter twist rate does add more resistance and increases chamber pressure.
    When testing new FN made M-16A2s against older Colt M-16s, ( a 1 in 7 twist rate compared to the old 1 in 12 twist rate ), the tighter twist rate showed an average of 50 to 70 fps less velocity using both M-855 and M-193 ammo.

    Though we used 5 rifles of each for our chronograph test, we still wondered if it may have been because the older Colt M-16s had very smooth barrels from extensive use as training rifles. While the FN Made M-16A2s were brand new.

    In your case there will not be enough difference to matter.
    If you were pushing very hot hand-loads, (like we do with the 6.8mm SPC) then you might want a looser twist rate.
  7. Magoo

    Magoo Well-Known Member

    Nailed it.

    Bullets are pretty easy to spin down their long axis, so a faster twist won't constitute a measurably significant percentage of the total energy of the system.

    But ya, there is some loss there, there is more friction between the bullet a barrel so more work has to be done to move it through there. It just may be less than the difference between two same-twist barrels from different manufacturers, or variables presented in the field.

    But I might be wrong :eek:
  8. rcmodel

    rcmodel Member in memoriam

    Considering that the bullet has the rifling engraved on it and is up to nearly full rotational speed by the time it has moved one bullet lengh into the rifling?

    Not much.

  9. jmr40

    jmr40 Well-Known Member

    The different twist may well have accounted for the difference. Especially since it was consistent with several rifles, but 20-50 fps difference is common between various rifles shooting the same ammo through equal length barrels with the same twist. I've seen well over 100 fps difference between the fastest rifle and the slowest I've tested.

    I'd really never thought about it, but shooting slightly slower through a faster twist does make sense. But I believe it is just one of several factors that effect velocity and the difference is probably not enough to worry about.
  10. Samclrk

    Samclrk Well-Known Member

    Then just make ALL barrels with a two or three twist and not worry with which is best a ten or eight twist..
  11. allaroundhunter

    allaroundhunter Well-Known Member

    Um, what?
  12. Samclrk

    Samclrk Well-Known Member

    Why then did Winchester and Remington and every body else build fourteen,sixteen twelve,and ten twist barrells ..Why not just build two or three twists and be done.
  13. allaroundhunter

    allaroundhunter Well-Known Member

    Because in theory, each different bullet weight will shoot best out of a specific rifling twist. For example, the optimum twist for a 55 gr is not the optimum twist for a 69 gr when maximum accuracy is desired.
  14. Samclrk

    Samclrk Well-Known Member

    Thank you ,Sir.
  15. Owen Sparks

    Owen Sparks member

    All things being equal a tighter twist does increase pressure. The twist rate for the 6.8 SPC II barrels has been slowed to allow to burn more powder and achieve a faster velocity.
  16. 68wj

    68wj Well-Known Member

    Since I saw 6.8 SPC mentioned a few times here, I thought this would be relevant to the conversation. Perhaps second only to .223/5.56, the 6.8 has been the subject of much debate regarding twist and its affect on velocity and pressure. The following was generated by a 6.8 barrel maker currently producing 1/7 twist barrels to stabilize 200 gr .277 bullets, but still reports to function well with the commonly found lighter loadings (he is also a member here and might be able to add to the discussion). This assumes that all is equal between barrels except the twist rate.



    I was able to use one of these 1/7 barrels recently with 100 grain handloads that I had worked up for a 1/11 barrel. No chronograph was present, but on paper at 100 yards, and steel at 200, I didn't see anything wrong and the brass looked fine.
  17. michael5446

    michael5446 Member

    simple physics, just depends how you frame it... it all has to to do with the coefficient of friction, the more more resistance you apply(ie more twist) the more back pressure and lower speeds....

    hope that helps

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