Twist rate is the distance a barrel’s rifling takes to complete one full turn. Faster twists use smaller numbers, such as 1:7, while slower twists use larger numbers, such as 1:12. Stability depends on several factors, especially bullet length, diameter, weight, velocity, and barrel twist.

This calculator uses the simplified Miller stability formula to estimate gyroscopic stability and also shows a Greenhill twist recommendation as a rough cross-check. Both are approximations and do not replace live-fire testing or advanced ballistics software.

Enter muzzle velocity, bullet diameter, bullet length, bullet weight, and your barrel twist. The calculator computes the bullet length in calibers and the twist in calibers per turn internally for the Miller formula.

Important: Both the Miller and Greenhill formulas are simplifications that assume standard bullet materials and forms. Results are approximations for comparison only and do not account for individual variations in ballistic coefficients, air density, spin decay, or other complex phenomena. Always verify with live-fire testing or advanced ballistics software before making critical decisions.

For a 62 grain .224 bullet, 0.900 inches long, fired at 3100 fps from a 1:9 twist barrel, the simplified Miller formula estimates SG ≈ 1.54 under standard assumptions. This result is model-dependent and sensitive to bullet form factor, which this calculator does not specify. The Greenhill estimate (≈1:10.0 at this velocity) serves as a rough cross-check. In practice, real-world stability depends on multiple factors beyond these approximations, so both results should be treated as guidelines rather than definitive answers. Field testing is essential.