1. What is the Stability Factor?

The stability factor (SG) is a dimensionless number that indicates how stable a projectile is in flight based on its twist rate, dimensions, and mass. Higher values indicate greater stability.

2. How Does the Calculator Work?

The calculator uses the stability factor equation:

Where:

• \( twist \) — Twist rate (calibers per turn)
• \( d \) — Projectile diameter (meters)
• \( \rho_{air} \) — Air density (kg/m³, typically 1.225 at sea level)
• \( l \) — Projectile length (meters)
• \( m \) — Projectile mass (kg)

Explanation: The equation accounts for how the projectile's rotation (from twist rate) counteracts destabilizing forces during flight.

3. Importance of Stability Factor

Details: A stability factor between 1.3 and 2.0 is generally considered optimal. Below 1.0 indicates instability, while values above 2.5 may indicate excessive spin.

4. Using the Calculator

Tips: Enter all values in the specified units. For standard conditions, use 1.225 kg/m³ for air density. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is a good stability factor value?
A: Most projectiles perform best with SG between 1.3 and 2.0. Values below 1.0 indicate instability, while above 2.5 may cause excessive wear.

Q2: How does air density affect stability?
A: Higher air density (lower altitude/colder temperatures) requires more spin for stability. Standard sea level density is 1.225 kg/m³.

Q3: What units should I use?
A: Use meters for dimensions, kg for mass, and calibers/turn for twist rate (1 caliber = bullet diameter).

Q4: Why is length squared in the equation?
A: Longer projectiles experience greater destabilizing forces, requiring more spin to maintain stability.

Q5: Does this work for all projectile shapes?
A: The formula works best for standard bullet shapes. Very unconventional designs may require additional factors.