# Greenhill Formula for Optimal Rifling Twist Rate

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AndrewBudd.Greenhill Formula for Optimal Rifling Twist Rate
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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:

• (D) the bullet's diameter
• (L) the bullet's length
• (SG)  the specific gravity of the bullet's material. Note: a default of 10.9 is provided for the SG of a lead-cored bullet.
• (V) the muzzle velocity of the average round.

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.