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# Greenhill Formula for Optimal Rifling Twist Rate

AndrewBudd.Greenhill Formula for Optimal Rifling Twist Rate

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.

### More Info

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

**For more interesting firearms calculators see:**

#### References

Further Information: