Fast Twist VS Slow Twist

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by jwp475, May 1, 2008.

  1. jwp475

    jwp475 Well-Known Member

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    I have a friend that works in a Ballistics lab and he ran a test with a 223 Rem with a 7 twist and compaired the pressure and velocity to a 14 twist. After runnig the test he told me that the difference in velocity and pressure between the 7 and 14 twist was insignificant.. Many claim that by running a slow twist that they can increase velocity without increaseing pressure, but judging by this test that is not the case. A fast twist barrel can degrade the jackets and cause problems with some bullet, but not all bullets, such as the Barnes TSX bullets..
     
  2. BountyHunter

    BountyHunter Writers Guild

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    That is certainly interesting.

    Cannot wait to see the article discussing it in detail because it certainly flys in the face of practice and experience to say "insignificant velocity and pressure between a 7 and 14 twist". Assuming same load and bullet were used in the same chamber and certainly not a copper solid Barnes.

    Until that article is published by Ty and subjected to a little peer review, I will stick with what I have proven to myself by firing the same loads in different twists. Seen too many guys pushing bullets in slow twists at velocities not possible with standard fast twists.

    But then maybe I and many others have been wrong all along. Would not be the first time. Guess we will wait for the article.

    BH
     

  3. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

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    I have had detailed personal conversations with Dan Lilja about this very subject. His feelings were basically the exact same as what your friend told you he saw.

    That said, while in a 223 or 308, you will likely not see much difference in pressure or velocity change but when you step up to a chambering that is really extreme, this insignificant change in pressure can become quite significant.

    For example, and this is only an example, NOT TRYING TO SELL RIFLES :D, if I took my 7mm AM with a 160 gr Accubond loaded to 3550 fps, which is a top end practical load, in a 1-10 twist Lilja barrel and then shoot that same exact load in a 1-7 twist Lilja barrel, the faster twist barrel will show obviously higher chamber pressures.

    Now if I take that same 7mm AM, load it to 3400 fps and repeat the test, you will hardly notice a difference between the two.

    Point being, if you run an extreme round to the red line, you will see MUCH more diffence when small changes are made in things such as bullet weight, twist rate, groove number, groove width and even things such as jacket thickness. In most chamberings these things are pretty much "insignificant" as far as increasing pressure but in extreme performance chamberings, that is not the case.

    Another good reason not to load to red line!!!

    For conventional chamberings loaded to conventional pressures with conventional weight bullets, I would agree 100%. With the extreme end of chamberings, it can be more noticable when you make changes like this.

    Kirby Allen(50)
     
  4. davewilson

    davewilson Well-Known Member

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    i would say the 7AM is an example of extreme overbore and the closer you are to the "red line" i think you called it, small changes will make more noticable differences. i'd like to see the same experiment done with larger diameter bullets to compare the amount of change in V and P to the smaller diameters to see if diameter would have any influence on the percentage of change.
     
  5. goodgrouper

    goodgrouper Well-Known Member

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    You guys can read something while you're waiting for the new article. Pick up a copy of Sierra's latest reloading manual and flip to the section about the 223 tests in differing twists. They found very insignificant results in running tight twists vs. slow twists as far as velocities were concerned.
     
  6. jwp475

    jwp475 Well-Known Member

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    gg, many will not let facts stand in the way of preconceived beliefs. Your signature is spot on...
     
  7. Mikecr

    Mikecr Well-Known Member

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    Thanks for sharing the info
    You know, every little bit helps