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`TwistRate = (C* "D" ^2) / "L" * sqrt(( 10.9 )/10.9)`

Enter a value for all fields

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.

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: http://www.ammoland.com/2017/02/ar-15-barrel-twist-rates-you-need-to-know/#axzz4alKeEk8o

**Sectional Density**: computes the factor used in the computation of the ballistic coefficient called sectional density.**Miller Twist Rule**: calculate the optimal barrel twist rate for stabilizing a bullet's flight given the bullet's diameter(D), length(L) and mass (m).**Greenhill Formula for Optimal Riffling Twist Rate**: 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).**Ballistic Range**: computes the maximum range (horizontal distance) traveled by an object based on the initial velocity (V) of the object, and angle of launch (θ), the launch point's height (h) above the plane**Ballistic Coefficient**: factor representing the effect of air resistance on a ballistic projectile.**Muzzle Energy of a Projectile**: calculate the kinetic energy in a bullet immediately after leaving the barrel given the bullet's mass and velocity.**Recoil Velocity of a Gun**: computes the velocity at which a gun will move in the opposite direction in relation to the projectile that it fired.**Cost per Round**: cost per round of ammunition based on the cost of a container of cartridges and the number of rounds in the container.

Further Information: