That Greenhill formula originated in the late 1800's. C.E. Harris, writing in the 08/1983 issue of the American Rifleman, noted Greenhill's formula was developed before spitzer boattail bullets and high velocity cartridges. He used a more modern analysis of gyroscopic stability, in which a factor of 1.4 is minimum and 1.7 is usually good. He found that the numbers given by Greenhill's original formula ranged from 1.5 to 2.0 for military type boattail bullets and were about 2.0 for bullets with either a flat base or short boattails.
While oft times claimed to be "the" standard for calculating twist rates for bullets, the Greenhill formula's been superceded by more accurate methods. For example, Sierra Bullets uses the time of flight between two points testing their bullets, The twist that ends up with the lowest spread in time of flight for a given bullet best balances it so its drag (ballistic coefficient) numbers have the lowest spread.
For decades, those "in the know" (competitve shooters producing the best results) have used the slowest twist that stabilizes bullets all the way to the target. The following twist rates for the 308 Win. cartridge shooting spitzer bullets of different weights leaving at typical muzzle velocities:
240 grain, 1:8
220 grain, 1:9
200 grain, 1:10
185 & 190 grain, 1:11
168, 175 & 180 grain, 1:12
150 & 155 grain (short barrels and ranges), 1:12
150 & 155 grain (long barrels and ranges), 1:13
135 grain, 1:14
For a wide range of bullet weights, it's best to use the twist that best stabilizes the heaviest bullet used. Which is why factory sporter barrels have the twists used; 1:10 for a .30-06, for example as it will handle bullets up to 220 grains quite well and lighter ones at 150 grains will still perform good enough for general use.
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Last edited by Bart B.; May 9, 2013 at 07:28 AM.