Does a short barrel need a faster twist?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Kronberg, Oct 14, 2009.

  1. Kronberg

    Kronberg Member

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    Hi

    We all know that a longer bullet needs a faster twist to stabilize the projectile. A longer barrel can spin a bullet longer. Is it thus able to achieve the same stabilization as a shorter barrel with a slightly slower twist?


    Any experience with it?
     
  2. BountyHunter

    BountyHunter Writers Guild

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    It is not the spinning of the bullet longer time. It is RPMs, which is all twist and has nothing to do with barrel length per se.

    So long or short it does not matter, it is the twist as long a MVs stay in the same ballpark. Change MV dramatically though, and it will change the need for twist changes.

    Perfect example is the 240 SMK.

    Sierra says it needs a 1-9 because it was originally designed for the MTU shooting 1000 yds at 2650 fps. Push it to 2850-2900 plus, and guess what, will shoot like a house afire in a 1-10. Drop the 240 SMK in a Whisper (221 fireball necked up to 30 cal) and the MV is down to 950 fps and you need to go to a 1-8 twist.

    BH
     

  3. Kronberg

    Kronberg Member

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    Hm, interesting.


    The barrel length thus influences the needed spin through the increased velocitiy. The 1-11 twist near the crown of a 26" barrel will spin the bullet accordingly a little bit faster than the one near the crown of a 20" one.

    Did one of the members here experience an sudden opening up of a group of heavy (long) bullets shot out of a barrel with relative low twist at a longer range compared to range shorter by 50-100 yards?
     
  4. Mikecr

    Mikecr Well-Known Member

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    Twist requirements are displacemnt per turn. For your example 11:1, thats 11" of displacement per turn. Doesn't matter the barrel length as this turn rate is just what it is, regardless of velocity.

    Also, bullets are normally gyroscopically stable, or they aren't.
    So if you make it 50yds without wacking paper sideways, it's gyroscopically stable(marginal, perfect, or otherwise).
     
    Last edited: Oct 14, 2009
  5. Michael Eichele

    Michael Eichele Well-Known Member

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    In response to your first comment yes. But only in the sense that the increased velocity is due to the longer barrel. All else being equal such as equal velocity and equal twist regardless if it was fired from a 17" barrel or a 30" barrel the stability factor will be identical or scary near identical.

    The second comment or question is not that simple. You can have a negative impact on downrange accuracy from both a bullet with a very low stability factor or a very high one. That statement is made through experience and is NOT scientifically NOR mathematically based so take it with a grain of salt. As MikeCr brought out, if a bullet doesnt hit the target sideways at 50 or 100 yards, it is stable be it marginal (low SF) or perfect (optimum SF) or otherwise (high SF). Regardless, it is gyroscopically stable. Which spectrum of SF a bullet is in can affect close, mid point and long range accuracy yes.

    Not sure if that answers your questions.
     
  6. J E Custom

    J E Custom Well-Known Member

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    Once a bullet is engraved barrel length does not matter except in velocity.

    A 10" barrel with a 1 in 12 twist will work fine to stabilize a bullet even though it does not
    make a complete revolution as long as velocity is high enough.

    J E CUSTOM
     
  7. kiwi3006

    kiwi3006 Well-Known Member

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    I was speaking to a guy once who shortened a barrel so he could fit a suppressor on it. Once it had been shortened the accuracy decreased. He put it down to the projectile doing less than one and a half revolutions while in the barrel. ie a 1 in 12 twist needs to be 18 inches long to get 1.5 revolutions in the barrel.

    Anyone else heard of this? Or was it more likely a poor crowning job when the barrel was shortened?

    Stu.
     
  8. J E Custom

    J E Custom Well-Known Member

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    I would be more inclined to believe that it was the crown or more likely when he shortened it
    the harmonics changed and he should have worked up new loads.

    Also by shortening the barrel he lost some velocity and should have increased the powder
    charge or changed powder in order to reach the "OLD" velocity.

    Lots of rifles shoot there best at just below the max pressure and velocity so if a rifle shoots
    well the best thing to do is chronograph the existing load and after changing/shorting the
    barrel work up new loads that match.

    I learned what little I know from a master gun smith that shot nothing but bench rest and he
    actually re crowned until he found the best harmonics (Shot the best). I know of one rifle that
    he re crowned 11 times before he was satisfied.

    J E CUSTOM