Miller Stability Formula
Also: Miller Twist Rule
A modern rule that estimates a bullet's gyroscopic stability factor from its length, diameter, weight, the barrel twist, and the muzzle velocity.
The Miller stability formula is a modern method for estimating the gyroscopic stability factor of a bullet, introduced by Don Miller as a more accurate replacement for older rules of thumb. It works from the projectile’s length in calibers, its diameter and weight, the barrel’s twist rate, and the velocity, then applies corrections for muzzle speed and for air density so the estimate holds across conditions.
Shooters generally look for a stability factor of roughly 1.4 or higher to be confident a load will fly cleanly, and the Miller rule makes it easy to check before buying components. It is more reliable than the older Greenhill formula because it accounts for bullet weight and velocity rather than length alone, which matters for the long, low-drag projectiles favored at extreme range.