The Greenhill Rifling Formula is used to calculate the optimal barrel twist rate for stabilizing a bullet's flight given the bullet's diameter(D), length(L), specific gravity (SG) and velocity(V). The resulting twist rate is the length in which the rifling of the barrel will make one full rotation.
INSTRUCTIONS: Choose units and enter the following:
Optimal Barrel Twist Rate (TR): The calculator computes the length (TR) for one complete rifling rotation in inches. However this can be automatically converted to other length units via the pull-down menu.
The Twist Rate is commonly written as a ratio. Some common rifling ratios are 1:7, 1:9, 1:36, where 1:7 corresponds to 1 turn in 7 inches.
The constant C corresponds to the velocity at which the bullet is fired. For velocities up to 2800 ft/s 150 is used. For anything higher 180 is used.
Here is an interesting article on why proper bullet stabilization matters: