barrel twist confusion

Discussion in 'Competition Forum (All Calibers)' started by Bruce W, Sep 13, 2018.

  1. Bruce W

    Bruce W Silver $$ Contributor

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    Jul 9, 2008
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    KWK, Berger 7mm 130 SMK at 2400 fps

    kwk = 10.6

    Berger = marginal stability with 10.6 and suggests a 9.25 twist

    Seeking target accuracy to 200 yds. Your comments and wisdom please
     
  2. mikecr

    mikecr

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    3,192
    Berger recommended based on ICAO air density. This is sea level 29.92"Hg, 59degF, 0%Rh.
    You can consider expected air density conditions w/resp to this & re-run the stability numbers.
     
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  3. damoncali

    damoncali Gold $$ Contributor

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    If I’m not mistaken, Berger also tries to get you to a 1.5ish Sg, which according to Bryan Litz’s experiments is *approximately* what you need to minimize yaw and get that last tiny bit of BC. At 200, you don’t care about that and can experiment with lower Sg values. I would go with “marginal” if it were my rifle. And definitely put in reasonable atmospherics. That matters. The deafault is usually pretty conservative.

    Also note that the Miller formula (which is what the Berger calculator uses) does not work as well for flat based bullets as for boattailed bullets if you want to investigate those.
     
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