Rifle Twist Rate & Bullet Stability Calculator

This ballistic calculator uses the Miller Twist Rule with atmospheric corrections for temperature, altitude, and muzzle velocity. The calculator also features a "subsonic" mode with built in correction for long aspect projectiles, such as 200+ gr 300 Blackout. Stability factors below 1.0 indicate tumbling; 1.5–2.5 is the optimal range for most precision applications.

Projectile Characteristics
Environmental & Velocity Factors
?
Automatic (Mach < 1 & ℓ > 4.0)

The standard Miller formula assumes supersonic velocities and overestimates stability for subsonic projectiles.

This correction activates automatically for subsonic, long-aspect projectiles but can be manually toggled for edge cases.
1:11
slower spin — 1:24 1:7 — faster spin
unstable
< 1.0
marginal
1.0–1.5
stable
1.5–2.5
over
> 2.5

Sg =

—

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Enter projectile parameters and set the twist rate above.

ℓ = length ÷ diameter standard conditions