Some time ago someone asked about how to determine the proper twist for a given caliber bullet when the length and weight is known. Well, this is it. 

 

The Greenhill Formula is used to determine the slowest rifling twist to stabilize a bullet.  

Divide 150 (constant) by the bullet length (expressed in calibers) and multiply that by the bullet diameter (expressed in inches).  

EXAMPLE:  

Use a .257 cal. 120-gr. bullet 1.125 inches long, divide its length by its diameter, which gives us 4.38 caliber. Next divide the constant (150) by caliber's (4.38) and you get 34.25. Multiply 34.25 by .257 (bullet diameter) which gives a twist of 8.80 or to the nearest twist of one turn in nine inches.