Bullet weights and Twist Rates

Discussion in 'The Ammo & Reloading Forum' started by FTK87, Dec 16, 2009.

  1. FTK87

    FTK87 New Member

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    I have been wondering for a while now, is there any formula to calculate which bullet weights will work well with what barrel twist rates for a given caliber?

    The information I am really wanting,is:

    Which bullet weights will perform in my .223 with 22" barrel with 1-12" twist

    Which bullet weights will perform in my .223 AR with 16" barrel with 1-9 twist

    and which bullet weights will perform in my .30-06 with 24" barrel and 1-12 twist?

    That is the info I am looking for, but would like to find a formula to calculate it for calibers and configurations in the future.

    Thanks
  2. JLA

    JLA Well-Known Member

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    Twist = 150 X diameter^2 / bullet length

    All measurements to be taken in inches with a dial caliper.

    Its known as the greenhill formula, I just happen to have a link to an online calculator;)

    http://kwk.us/twist.html Enjoy:)
  3. FTK87

    FTK87 New Member

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    If I'm not mistaken, that tells you which twist rate a particular projectile needs, right?

    I don't have any of my bullets handy for a test.

    Would you be able to use any bullets lighter than the ones the formula calls for?

    What is the limit you can push as to how much heavier or lighter your projectile can be and still effectivly stabilize?
  4. new308handloader

    new308handloader New Member

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    What is the "SG value" in that calculator? 10.9 is the default?
  5. FTK87

    FTK87 New Member

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    It shows if you look just below the calculater, that is for what material the projectile is made of.
  6. new308handloader

    new308handloader New Member

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    Yes, but I still don't see why 10.9 is in the field when that is not a choice.

    Here are the 4 choices listed:
    11.3 lead
    8.9 copper
    8.5 brass
    7.8 steel

    Is it that (10.9) is the value of a "normal" bullet that is made of both copper and lead? I have just never seen an "SG" value for a bullet and not sure where you would get that number for the specific bullet you are using?

    Just trying to understand.....I want to use this calulator once I know my "SG" value is for the bullets I use....I appriciate the link.
    Last edited: Dec 16, 2009
  7. LurpyGeek

    LurpyGeek Active Member

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    JLA provided the best info. Appropriate twist rate is really about how long a projectile is, but most bullets are differentiated depending on what weight they are. More weight = longer bullet. The length of your barrel with have an impact on the muzzle velocity of the departing projectile (longer barrel = longer push from gas pressure = higher velocity), but won't really make a difference in accuracy.

    I can tell you from my own experience that your 1-in-12" .223 will be best around 55 grain and lower. the 1-in-9" .223 should be quite versatile from 45 grain all the way up to the high 60 grain range. I know some people who have shot 77 grain in a 1-in-9" with great accuracy, and others who started to see spreading groups. All depends on your barrel, and you won't really know what will work until you try it.

    Just out of curiosity, are the barrels steel, stainless, or chrome lined?

    I can't really comment on the .30-06 from first hand experience, but I'm sure others will be able to (or Google).
  8. FTK87

    FTK87 New Member

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    all the barrels are steel, I wish the AR was chrome lined, but its not.
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