Twist Rate Calculator
Calculate rifle barrel twist rate for bullet stabilization using Miller's rule or Greenhill's formula
Calculate Barrel Twist Rate
Weight of the bullet projectile
Diameter of the bullet (e.g., 0.308" for .308 caliber)
Overall length of the bullet
Initial velocity of the bullet leaving the barrel
Target stability factor (2.0 is safe, 1.5 minimum for accuracy)
Environmental Conditions
Twist Rate Results
Method: Miller's Rule
Formula: t² = 30m / (s × D³ × l × (1 + l²))
Where: m = 0.0 grains, D = 0.000", l = NaN calibers, s = 2
Very stable - Excellent accuracy potential
Common Caliber Twist Rates
Example Calculation
.308 Winchester Example
Bullet: 168 grain HPBT
Diameter: 0.308 inches
Length: 1.20 inches
Velocity: 2650 fps
Stability Factor: 1.8
Miller's Calculation
Length in calibers: 1.20/0.308 = 3.90
t² = 30×168 / (1.8×0.308³×3.90×(1+3.90²))
t² = 5040 / (1.8×0.0293×3.90×16.21)
t = √(172.8) = 13.15
Twist Rate = 1:11.6"
Stability Factor Guide
Unstable
Bullet will tumble in flight
Marginally Stable
May experience accuracy issues
Adequately Stable
Good for most applications
Very Stable
Excellent accuracy potential
Rifling Tips
Faster twist rates stabilize longer, heavier bullets
Miller's rule is more accurate than Greenhill's formula
Higher velocity requires slightly faster twist
Environmental conditions affect bullet stability
1:8" is faster than 1:12" (more rotations per distance)
Understanding Barrel Twist Rate
What is Twist Rate?
Twist rate refers to the distance a bullet travels to complete one full revolution along its longitudinal axis as it passes through a rifled barrel. It's expressed as a ratio, such as 1:10", meaning the bullet completes one full rotation in 10 inches of barrel length.
Why is Rifling Important?
Rifling imparts spin to the bullet, creating gyroscopic stability that prevents tumbling in flight. This dramatically improves accuracy and effective range compared to smoothbore firearms. The proper twist rate ensures optimal bullet stabilization for the intended projectile.
Miller's Rule vs Greenhill's Formula
Miller's rule is more accurate and accounts for bullet shape, while Greenhill's formula is simpler but less precise. Miller's rule also allows for environmental corrections and is preferred for modern ballistics calculations.
Miller's Rule Formula
t² = 30m / (s × D³ × l × (1 + l²))
- t: Twist rate per caliber
- m: Bullet mass in grains
- s: Gyroscopic stability factor
- D: Bullet diameter in inches
- l: Bullet length in calibers
Greenhill's Formula
t = C × D² / (L × √(SG/10.9))
- C: Constant (150 or 180 for high velocity)
- D: Bullet diameter in inches
- L: Bullet length in inches
- SG: Specific gravity of bullet material
Velocity Correction
For velocities > 2800 fps:
f_v = (v/2800)^0.33
Higher velocities require slightly faster twist rates.
Altitude Correction
For elevation effects:
f_h = e^(3.158×10^-5 × h)
Higher altitudes reduce air density, affecting stability.
Temperature Correction
For temperature and pressure:
f_t = (P_std/P_T) × (T+460)/519
Temperature affects air density and bullet performance.