Date: 07 Aug 94  18:31:02
From: Henry Niemi
To:   Brooks Gelfand
Subj: Greenhill formula

Greenhill's Formula for Required Twist to stabilize bullets:
(Hatcher's Notebook, 3rd edition/2nd printing, p. 556)

                                     (Bullet length)
Required Twist (in calibers) = 150 / ---------------
                                     (Bullet caliber)

Ex: Bullet = 220 grains, .30 caliber, length=1.35 in
    cal = 1.35/.30 = 4.5
    Req. twist in cal. = 150/4.5 = 33.33 in cal.
    Req twist = 33.33 x (.30 cal.) = 9.99, or about 10" (1/10 twist)

NOTES (also from Hatcher's book):

1.  Formula is applicable to bullets with specific gravity of 10.9
    (most jacketed lead bullet are close to this value). Lead itself has
    a specific gravity of 11.35.

2.  For bullets made of materials other than lead, twist rate should be
    multiplied by the square root of (10.9 / S.W. of material used). Ex:
    Aluminum (SW = 2.7) bullet, same dimensions as above.
    sqr root (10.9/2.7) = sqr rt (4.03) = 2 approx.
    Req. twist = 9.99 x 2 = 20, or 1/20 twist.
    An Aluminum bullet would require "TWICE MORE" spin as a jacketed lea
    one. (It seems like TWICE LESS twist to me).

3.  Formula is applicable to bullet flight in air. In denser
    materials, "MORE" spin is required to stabilize the same bullet.
    Twist in density 2 = Twist in air x sq rt (density 2 / air density)
    Ex:  Water has density 900 greater than air: Dwater/Dair = 900
         sq rt (900) = 30
         Twist rate in water = 10 x 30 = 300 , or 1/300.
         (Looks like LESS spin to me, not MORE).
------------------------------------------------------------------------

Hatcher's / Greehill terminology is confusing.  From the Hatcher book,
it says that a less dense bullet requires MORE spin, yet the way Hatcher
presents his calculations, it actually require LESS spin (1/20).

Likewise for a denser medium: it says that in H2O, the same bullet would
require MORE spin than in air, yet the actual calculations per Hatcher's
instructions show that LESS spin is required (1/300).

Can anyone help clarify that ?  Thanks.

--- WILDMAIL!/WC v4.00*
 * Origin: The Rising Storm BBS in California, 408-379-4329,  (1:143/220.0)

