This calculator helps determine the stability of a bullet based on its rifling twist rate and other ballistic factors. A stable bullet flies true, maximizing accuracy and downrange performance. An unstable bullet will tumble or keyhole, significantly reducing its effectiveness.
Understanding Bullet Stability (The Greenhill Formula)
The Greenhill formula is a widely used empirical formula to estimate the required rifling twist rate for a given bullet to achieve stability. It's a good starting point, though modern ballistic calculators may incorporate more complex aerodynamic factors.
The simplified concept behind twist rate stability is that a spinning bullet is inherently more stable, much like a spinning gyroscope resists falling over. The faster the bullet spins, the greater its gyroscopic stability, provided the spin rate is sufficient to counteract aerodynamic forces that would otherwise cause it to tumble.
The formula can be rearranged to calculate a Stability Factor (SF). A generally accepted threshold for stability is an SF of 1.0 or greater.
Key Factors:
Bullet Diameter: A larger diameter generally requires a slower twist rate for the same length.
Bullet Length: Longer bullets are less stable and require faster twist rates. This is a critical factor.
Bullet Weight: Heavier bullets tend to be more stable, but length is usually more dominant.
Rifling Twist Rate: This is the rate at which the rifling inside the barrel completes one full turn. Expressed as "1 in X inches", where X is the number of inches for one full rotation. A lower number (e.g., 1:7) means a faster twist.
Bullet Velocity: Higher velocities increase the centrifugal forces acting on the bullet, generally enhancing stability up to a point.
Air Density: Denser air at lower altitudes or temperatures can exert more force on the bullet, affecting stability.
Where 'C' is a constant, often around 150 for standard lead core bullets at typical velocities. However, for calculating a Stability Factor, we use the actual twist rate and other parameters.
Stability Factor (SF) Calculation:
The calculation performed by this tool is a variation of the Greenhill formula to provide a Stability Factor (SF).
SF = (300 * Diameter²) / (Bullet_Length * Twist_Rate_Ratio)
Where:
Diameter is in inches.
Bullet_Length is in inches.
Twist_Rate_Ratio is calculated from your input (e.g., if you input "7" for 1:7 twist, the ratio is 7).
This simplified SF calculation is a good indicator. Many advanced calculators also consider velocity and air density, as they influence aerodynamic forces. Our calculator integrates these for a more refined estimate.
Note: This is a theoretical calculation. Actual bullet stability can be influenced by manufacturing tolerances, bullet construction (e.g., boat tail vs. flat base), atmospheric conditions, and the precise aerodynamic properties of the projectile.