# Greenhill Formula for Optimal Rifling Twist Rate

Not Reviewed
Equation / Last modified by AndrewBudd on 2017/03/09 16:34
TwistRate =
Greenhill Formula for Optimal Rifling Twist Rate
Variable Instructions Datatype
(D)"Diameter" Diameter of the bullet. Decimal (in)
(L)"Length" Length of the bullet. Decimal (in)
(SG)"Specific gravity" 10.9 for solid lead core bullets. Decimal
(V)"Muzzle velocity" Velocity at which the bullet is fired. Decimal (ft/s)
Type
Equation
Category
vCommons
Contents
4 variables
Rating
ID
AndrewBudd.Greenhill Formula for Optimal Rifling Twist Rate
UUID
fa6a549f-0929-11e5-a3bb-bc764e2038f2

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. The resulting twist rate is the length in which the rifling of the barrel will make one full rotation.
INSTRUCTIONS: Choose your preferred 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.

The calculator computes the length (TR) for one complete rifling rotation in inches.  However this can be automatically converted to other lenght 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 math see Muzzle Energy of a Projectile

#### References

Further Information: