ORIGINAL: charlie brown
How's this?
for a .30 cal. 180 gr. Spitzer bullet that is 1.35 inches long. here is the formula.
150 divided by length of bullet in calibers. Only applies to bullet with specific gravity of 10.9- jacketed rifle bullets.
1.35/.30= 4.5 calibers; 150/4.5= 33.33 calibers, or that many times the diameter of the bullet. 33.33 x .30= 9.9999 or 1 in 10" twist.
This came from Mastering Mule Deer by Wayne Van Zwoll, and was referred to as the Greenhill formula.
This is the Greenhill Formula, (developed by an instructor at the British Royal Artillery School in the late 1800's) as described in
Hatcher's Notebook:
Twist (
in calibers) = 150/length of bullet
in calibers.
It works for ALL regular jacketed bullets. But, as the example given by Charlie Brown shows, the answer will come out approximate, eg., you use a 1/10 twist for a bullet that requires a twist of 1 turn in 9.999 inches, etc. Naturally, a twist that will stabilize a bullet of a given length will also stabilize all bullets of that same caliber which ARE SHORTER!! Weight of the bullet does not enter into this formula, ONLY LENGTH of the bullet matters!