The interesting thing about the Greenhill formulas is while popular, top level competitive shooters use different twists for their bullets than what Greenhill calculates.
In 1879, Greenhill developed a rule of thumb for calculating the optimal twist rate for lead-core bullets in small arms. This shortcut uses the bullet's length, needing no allowances for weight or nose shape. Greenhill applied this theory to account for the steadiness of flight conferred upon an elongated projectile by rifling.
With modern bullets at the speed they leave at, it's not all that useful. The slowest spin rate that keeps a bullet stabilized all the way to the target is the best one to use. Any faster and the normal, slight unbalance of bullets (they're all unbalanced to some tiny amount) causes them to wobble too much increasing drag proportionally to their rpm's. Unbalanced bullets also jump off the bore axis as the leave the muzzle due to centrifugal forces and take a more spiraled flight to the target.
Last edited by Bart B.; October 9, 2012 at 06:49 PM.