The Greenhill Formula

The Greenhill Formula predicts the required barrel twist rate (number of inches for one complete rotation) to stabilize a given bullet, based on the bullet caliber and length. This formula is somewhat dated (it's over 70 years old), but still provides a good 'rule of thumb' for determining the required twist rate for proper bullet stabilization. The important thing to remember is that this formula is a good simplification of the more accurate, complex equations.

The formula is :

minimum required twist rate = (150 * bullet_diameter2) / bullet_length


It will quickly become clear to all who use this formula that the general rule is : the longer the bullet (i.e. in general, the heavier the bullet, in any given caliber), the quicker the twist required to effectively stabilize the bullet.


It is also quite simple to determine the longest bullet of a given caliber that a given twist rate will suitably stabilize by manipulating the formula :

maximum bullet length = (150 * bullet_diameter2) / twist_rate

The 'rec.guns.FAQ' reference below states that the '150' constant works well at velocities in the vicinity of 1500 fps or greater, while a value of 180 can be used with good results at velocities at 2800 fps. Using 180 as the constant in the equation will give you a 20% greater maximum bullet length, or a 20% slower required twist.



Twist Rate



                FASTER            MEDIUM              SLOWER

                1-in-7            1-in-10             1-in-12
                                               (1 turn in 12 inches)


Examples

  • 80 grain Sierra 0.224 = 1.064" in length

    required twist rate = (150 * 0.2242) / 1.064
    required twist rate = (150 * 0.050176) / 1.064
    required twist rate = (1 turn in) 7.07 (inches)


  • 150 grain Nosler 0.277 = 1.362" in length

    required twist rate = (150 * 0.2772) / 1.362
    required twist rate = (150 * 0.076729) / 1.362
    required twist rate = 8.45


  • 168 grain Sierra 0.308 = 1.210" in length

    required twist rate = (150 * 0.3082) / 1.210
    required twist rate = (150 * 0.094864) / 1.210
    required twist rate = 11.76


    'Twist Rate' Links



    Last updated : 09 November 1997