??? Gain, Progressive, Incremental, or Transitional Twist

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by rscott5028, Apr 20, 2011.

  1. rscott5028

    rscott5028 Well-Known Member

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    This came up in another thread...

    What do you think about Gain, Progressive, Incremental, or Transitional Twist?

    Pros and cons?

    Rule of thumb?

    Annectdotal or empirical evidence?

    thanks!
    richard
     
  2. Bart B

    Bart B Well-Known Member

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    All the top custom barrel makers agree the single most common cause of poor accuracy in an otherwise "perfect" barrel is a non-consistant twist. Some offer tests on your barrel's twist to see if there's any change in the rate as a tool's pushed through the bore; they'll send you a percentage number showing the amount of uneven twist (or something like that).

    Any change in twist rate will change the engraving angle on a bullet the rifling makes. And it won't do in uniformly all the way around the bullet. The bullet will then be more unbalanced than when it was when entering the rifling. Depending on how the jacket peels/deforms/resahaps when this happens, the result is displaced jacket material.

    Why unbalance a bullet any more than it already is?
     

  3. rscott5028

    rscott5028 Well-Known Member

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    Bart B,

    From a layman's perspective, I can relate to the difference between stop and go ("jitter" as we would call it in telecom)

    vs. a gradual/smooth accelerated twist.

    So, are you saying there's no benefit because it can't be done smoothly? Or, that it just adds complexity where it's not needed?

    thanks,
    richard
     
  4. MontanaRifleman

    MontanaRifleman Well-Known Member

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    Richard, Most guys are using gain twists to shoot long, high BC bullets requiring very tight twists, especially in the larger cals. Bartlein seems to think that the gain twists are OK. From what I've read there are some guys shooting them with good results, but I would hesitate to get one unless I really needed for the mentioned reason.

    -Mark
     
  5. Bart B

    Bart B Well-Known Member

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    It adds complexity and I don't think it's needed. I don't think there's any proof it is better that a constant rate twist.

    That aside, folks who believe in it can do so.

    It's the same as some folks knowing a Win. Model 70 is near three times as stiff as a long Rem. 700 one. Those who swear the 700's stiffer keep quiet after they dynamically measure one of each.
     
  6. TOM H

    TOM H Well-Known Member

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  7. Bart B

    Bart B Well-Known Member

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    I read that article.

    I don't understand why the 370-gr. bullet needs to be spun about 270,000 rpm when Sierra's 350-gr. MK needs only about 180,000 rpm to stabilize it at the same muzzle velocity. If it's the Rocky Mtn. one, their specs says it needs only about 200,000 rpm to stabilize it; and that's at 3100 fps which means it needs to be spun a tiny bit faster that at 2925 fps.

    Someone, please help me understand this. That gain twist barrel's rifling angle to the bullet goes from 4.2 degrees to 8.8 degrees. Seems to me that's a lot of jacket material that gets relocated.

    I'd like to see a 20-shot group fired at 600 yards with these bullets spun 280,000 rpm as they leave the barrel.
     
    Last edited: Apr 21, 2011
  8. TOM H

    TOM H Well-Known Member

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    Here some more reading

    http://www.longrangehunting.com/forums/f19/375-408-banded-solids-test-39553/
     
  9. MontanaRifleman

    MontanaRifleman Well-Known Member

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    Last edited: Apr 22, 2011
  10. Bart B

    Bart B Well-Known Member

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    I find it amazing, near unbelievable and surprising that gain twist barrel making has progressed to what the accuracy shown in these posts has been.

    I can imagine the centrifugal force on a bullet leaving at 3000 fps from a 6.5 twist spinning over 332,000 rpm is. After going to the Cutting Edge web site, it's clear as optical grade glass. I checked out the bullet design page and it was easy to see that gain twist rifling deforms only a small band around the bullet. The rest of the bullet's virtually unchanged by a 2X increase in twist rate.

    And those extra long bulletw will need a very fast twist, too. Being solid, they won't come apart spinning as fast as they do.

    Thanks for linking my mind to the modern bullets available.