Greenhill Formula and Twist.......

Discussion in 'The Ammo & Reloading Forum' started by 312shooter, Sep 6, 2008.

  1. 312shooter

    312shooter Active Member

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    Does anyone have some info on the greenhill formula that you have applied and found it accurate? I have googled it and found several different calculators. I'm not sure which to believe- I've got answers from 145gr all the way up to 220gr to stabalize properly in a 1/11 twist m1a scout, I think it would be easier to buy about 10,000 bullets of all weights, load them, and figure it out on the range! At least it would be more fun...thanks guys
  2. JLA

    JLA Well-Known Member

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    TWIST = 150(DIAMETER squared)/ LENGTH; length and diameter refer to the projectile in question... and yes i used this method to select the heaviest bullet that would stabilize properly from a 1 in 10" 03A3 barrel. and found it to be accurate and reliable...
  3. LDBennett

    LDBennett Well-Known Member

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    That is the correct formula of measurements in inches and it works.

    But you must have accurate measurements of the length of the bullets. Some manuals reproduce their bullet pictures 1 to 1 or if not, since you know the diameter you can scale the pictures. A real bullet would be better but buying a hundred bullets to measue one is probably not cost effective when you find the bullet too long for your twist rate.

    A 1.250 inch 168 gr match bullet is right at the limit (Maximum length) for a 1 in 11 inch twist, based on my calculations. Find a shorter one for more stability. But I would guess that your rifle would shoot that bullet fine, since it is commonly used in match 308 guns of unknown twists.

    LDBennett
  4. 312shooter

    312shooter Active Member

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    Thanks, I have read however that the "150" in that formula is used for projectiles under 1500 fps and to use 180 for over 1500, something to do with artillery vs small arms projectiles. If you guys found it accurate I'll trust you, thanks gents.
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