Miller Twist Rule

The Miller Twist Rule is used to calculate the optimal barrel twist rate for stabilizing a bullet's flight given the bullet's diameter(D), length(L) and mass (m) . 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) Bullet Diameter 
• (L) Bullet Length 
• (m) Mass of Bullet

Optimal Barrel Twist Rate (TR): The calculator computes the length  for one complete rifling rotation in inches.  However this can be automatically converted to other length units via the pull-down menu.

Firearm Calculators

• Sectional Density: Computes the factor used in the computation of the ballistic coefficient called sectional density.
• Bullet Ballistic Coefficient: Computes factor that represents a bullet's ability to overcome air resistance in flight.
• Ballistic Coefficient from Bullet's Mass, Diameter and Form Factor: Estimates the ballistic coefficient from the mass, diameter and form factor
• Miller Twist Rule: Computes the optimal barrel twist rate for stabilizing a bullet's flight given the bullet's diameter, length and mass.
• Taylor Knock-out Factor: Calculates a factor indicating the power of a round,.
• Greenhill Formula for Optimal Rifling Twist Rate: calculate the optimal barrel twist rate for stabilizing a bullet's flight given the bullet's diameter, length, specific gravity and velocity. 
• Bullet Flight Range: Computes the maximum range (horizontal distance) traveled by a bullet based on the muzzle velocity, elevation angle and shooter height.
• Muzzle Energy of a Projectile: Calculates 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: Computes cost per round of ammunition based on the cost of a container of cartridges and the number of rounds in the container.
• Rifle Sight Correction Angles: Computes the Minutes of Angle corrections for rifle sights.
• Shotgun Shell Reloading Cost: Computes the cost to reload used shotgun shells (hulls) with powder, shot, wads and primers.
• Shotgun Shell Loading Cost: Computes the cost to load new pre-primed shells (hulls) with power, shot and wads. 
• Metal Ball Weight: Computes the mass (weight) of a spherical metal (e.g., steel) ball based on the size (diameter).
• Metal Cylinder Weight: Computes the mass (weight) of a metal cylinder based on the size (diameter and length).

The Math / Science

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.

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

References

Further Information:

• https://en.wikipedia.org/wiki/Miller_twist_rule
• https://loadoutroom.com/thearmsguide/twists-fate-rifling/