Barrel Twist Rate & Bullet Stability Calculator
Will your barrel stabilize that bullet? Compute the Miller stability factor for any twist and load.
Miller stability formula. Aim for SG ≥ 1.5 for reliable stabilization across conditions; 1.4 is generally fine in warm weather. Faster twist (smaller number, e.g. 1:7) stabilizes longer, heavier bullets.
How it works
Barrel twist rate (e.g. 1:7, 1:9) determines whether your barrel spins a given bullet fast enough to stabilize it. Long, heavy bullets need a faster twist; short, light ones are fine with a slower twist. This calculator uses the Miller stability formula to estimate your gyroscopic stability factor (SG) from bullet weight, length, diameter, twist rate, and velocity.
Read the result like this: an SG of 1.5 or higher means the bullet is well stabilized, 1.0-1.4 is marginal (accuracy may suffer and it gets worse in cold, dense air), and below 1.0 means the bullet won't stabilize and will likely keyhole. It's the fastest way to know if a barrel will shoot the heavy bullets you want before you buy ammo — especially for AR-15 shooters choosing between 1:9, 1:8, and 1:7 twists.
Educational estimates only — always verify zero and data by shooting, and follow all safety rules.