Calculate rifle barrel twist rate stability using Greenhill and Miller formulas. Includes bullet RPM, stability factor, and twist comparison table.
Rifle barrel twist rate determines how fast a bullet spins as it leaves the muzzle. The rifling grooves impart a spin that gyroscopically stabilizes the bullet in flight — too slow and the bullet tumbles, too fast and it can jacket-separate or magnify manufacturing imperfections. The twist rate is expressed as "1 turn in N inches" (e.g., 1:7" means one full revolution every 7 inches of barrel).
The classic Greenhill formula from 1879 gives a quick approximation: T = Cd²/L, where C is 150 for subsonic or 180 for supersonic velocities. The modern Miller stability formula is more accurate, yielding a dimensionless stability factor (SG) where SG ≥ 1.3 is stable and SG ≥ 1.5 is preferred for consistent accuracy at long range.
Longer, heavier-for-caliber bullets need faster (lower number) twist rates. This is why modern military rifles use 1:7" twist for 62-grain 5.56mm ammo — the steel-core penetrator is longer than a lead-core bullet of the same weight, requiring faster spin. This calculator provides both formulas plus a full twist comparison table.
Shooters selecting barrels need to match twist rate to bullet weight and length. Handloaders testing new bullet designs need stability predictions before range testing. This calculator replaces manual Greenhill/Miller calculations. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation. Align this note with review checkpoints.
Greenhill: T = C×d²/L (C=150 subsonic, 180 supersonic). Miller SG = 30×W/(T²×d³×L×(1+L²)) where T, d, L in calibers. RPM = (velocity×12)/twist.
Result: SG = 1.82, RPM = 264,000
A 5.56mm M855 round at 940 m/s from a 1:7" barrel: the bullet makes 264,000 revolutions per minute. SG = 1.82, well above the 1.3 minimum — very stable.
Use consistent units, verify assumptions, and document conversion standards for repeatable outcomes.
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The distance the bullet travels down the barrel for one complete revolution of the rifling. A 1:7" twist means one turn every 7 inches. Lower numbers = faster spin.
A dimensionless number (SG) predicting gyroscopic stability. SG < 1.0 = unstable (tumbling). SG 1.0-1.3 = marginal. SG ≥ 1.3 = stable. SG ≥ 1.5 = well-stabilized.
The original M193 ball (55gr, short) was stable at 1:12. The M855 (62gr with steel penetrator, longer) needed 1:7 for adequate stability, especially in cold weather.
Yes — excessive spin can split bullet jackets ("spin failure"), magnify bullet imbalances, and slightly increase drag. But for most commercial bullets, this only happens at extreme twist rates.
Yes. An over-stabilized bullet (very high SG) will not "sleep" (precess to align with flight path) as quickly. Marginal stability (SG 1.0-1.3) causes yaw-induced dispersion.
Push a tight-fitting cleaning rod with a jag and patch through the barrel while watching the rod handle rotate. Mark the rod at the muzzle; the distance traveled for one handle rotation is the twist rate.