Re: Rifle barrel twist - can someone explain?
Greenhill's formula is the standard method for calculation the optimum twist for a given bullet.
Twist = 150 X Dē/L
Where: D = bullet diameter in inches L= bullet length in inches
This formula had limitations, but worked well up to and in the vicinity of about 1,800 f.p.s. For higher velocities most ballistic experts suggest substituting 180 for 150 in the formula. The twist formulas used in the "Load From a Disk" program uses a modified Greenhill formula in which the "150" constant is replaced by a series of equations that allow corrections for muzzle velocity from 1,100 to 4,000 ft/s.
You can search Wikipedia under twist rate for more information.
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