Twist Rate Calculator

The twist rate calculator can help you to find out how stable a bullet will be when shot from a specific rifled barrel. It's long been known that a bullet's longitudinal axis must spin or rotate in order for it to travel large distances while maintaining stability. The introduction of grooves into the barrel causes the bullet to spin. The Greenhill formula or the Miller twist rule are used to assess bullet stability when different barrels are used. To learn more about twist rate and how to calculate it, continue reading.

What is Twist Rate and How does it affect you?

A firearm's barrel is the most frequent component. The precision and range of weapons have come a long way from muzzle-loaded muskets with an effective combat range of a few hundred yards in the early 17th and 18th centuries to the current Barrett M82 with a maximum range of roughly 1900 yards. This advancement in technology has benefited artillery guns as well, with modern howitzers having a combat range that is around 20 times that of artillery guns used in the American Civil War.

Much of it is due to varied bullet forms that comply with aerodynamics in order to avoid tumbling in mid-flight and achieve improved stability. The bullet's stability has been increased by inducing spin along its longitudinal axis, resulting in a longer flight time and range. The barrel spins due to the torque exerted by grooves within the barrel.

On the other hand, Smoothbore artillery and tank guns do not have grooves. These weapons are designed to discharge kinetic energy projectiles that are significantly longer than their diameter. Specialized fins are used to stabilize such projectiles. Smoothbore barrels are less prone to wear and strain, and so have a longer lifespan.

Rifling is a term used to describe the process of creating grooves in a barrel. Twist rate is the metric used to define it. The distance traveled by a bullet to complete one full revolution is specified as the twist rate. 1 turn in x inches, or alternatively x inches per turn in English terms or 1 turn in y mm in SI units, is the unit of measure for barrel twist rate. The requisite rifle twist rate and bullet stability can be calculated using one of two formulas.

The Formula for Greenhill: Prof. George Greenhill first proposed the equation in 1879, and it is still used as a rule of thumb to determine the twist rate t in inches per mm.

t = C * D2 / L * √(SG/10.9)

  • Where;
  • C = A constant with a value of 150 or 180 (if the muzzle velocity exceeds 2800 ft/s)
  • D = Bullet/projectile diameter in inches
  • L = Projectile length in inches
  • SG = Specific gravity.

Miller twist rule: To estimate the bullet's stability, the rule employs a semi-empirical relationship. For a bullet with mass m and dimensional parameters such as diameter D and length L, the dimensionless gyroscopic stability factor s is: s = 30 * m / (t2 * D3 * l * (1 + l2))

Similarly, rearranging the terms yields the equation for uncorrected twist t, expressed as twist per one calibre: t2 = 30 * m / ( s * D3 * l * (1 + l2))

The value of s is initially set to 2.0, which is considered to be a safe amount for the stability factor. When multiplying the twist t from the Miller twist rule with the diameter of the bullet, you can get the twist rate in inches per turn T. The value of T is provided immediately by this calculator.

T = t * D

For parameters in English units, the Miller twist rule is valid, as follows:

The bullet's mass, m, is measured in grams, while its length L, and diameter D, are measured in inches.

How can I use the Greenhill Formula to calculate the Twist Rate?

Greenhill's formula for calculating twist rate is as follows:

  • Step 1: Divide the bullet's specific gravity, SG, by 10.9.
  • Step 2: Calculate the resultant's square root.
  • Step 3: Multiply the resultant by the square of the projectile diameter, D, and the constant C, 150.
  • Step 4: To get the twist rate in inches per turn, t, divide the product by the projectile's length L i.e. t = C * D2 / L * (SG/10.9)

What is the Miller Twist Rule's Velocity Correction Factor?

fv = (v/2800)1/3 is the velocity correction factor.

It's used to accommodate cases with increased muzzle velocity. For muzzle velocity greater than 2800 ft/s, the formula is valid.

How do you compute the Miller Twist Rule's Temperature Correction Factor?

In the Fahrenheit scale, the temperature correction factor is calculated as follows:

  • Step 1: Increase the temperature by 460 degrees Fahrenheit (in Fahrenheit).
  • Step 2: Divide 519 from the total.
  • Step 3: Multiply the value by ordinary atmospheric pressure to get the final result.
  • Step 4: Divide the result from the current atmospheric pressure.

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FAQs on Twist Rate Calculator

1. What is the formula for calculating twist rate?

Twist rate is defined as the number of complete 360-degree twists the rifling produces per inch of the barrel, for as 1:10-inch, which means one complete twist per 10 inches. The twist rate is unaffected by barrel length; a 1:10 twist rate stays 1:10 whether the barrel is three inches or three feet long.

2. Is a higher twist rate better?

Twist rate is the rate at which the rifle barrel spins, and it is measured in inches per turn. To stabilize the bullets you're shooting, you'll need a barrel with a sufficient twist rate. The twist rate, or how fast the bullet spins, is proportional to the number of twists.

3. Is it possible to overspin a bullet?

Due to centrifugal force, excessive spin can cause a bullet to explode as it exits the barrel. Rifling that is too tight might strip the case as the bullet passes through the barrel. Back to the state of overstabilization.

4. Is the twist rate affected by barrel length?

It's the rate, angle, or degree of the rifling twist that matters, not the linear distance. A 1:48" twist rate is defined as a groove that twists at a pace sufficient to make a full turn in 48 inches, independent of barrel length.

5. What effect does barrel twist have on velocity?

We discovered that muzzle velocity related to twist rate at a rate of around 1.33 fps per inch of twist when all the smoke cleared. In other words, going from a 1:12" twist to a 1:8" twist reduces your velocity by around 5 fps.