Calculate rifle barrel twist rate stability using Greenhill and Miller formulas. Includes bullet RPM, stability factor, and twist comparison table.
Rifle barrel twist rate describes how quickly the rifling spins a bullet as it leaves the muzzle. That spin helps keep the projectile point-forward in flight, and the right twist depends on bullet length, weight, diameter, and velocity.
This calculator uses the Greenhill approximation and the Miller stability formula to estimate whether a bullet is likely to be stable in a given barrel. It also reports bullet RPM so you can see how much spin the twist rate produces.
Use it when comparing factory barrels, choosing a twist for handloads, or checking whether a specific bullet length is a good match for the barrel you already have.
Use this calculator to compare twist rates before choosing a barrel or loading a bullet that is longer than what your current twist was designed for. It is most useful when you want a quick stability check without working through Greenhill or Miller by hand.
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 = 316,000
A 5.56mm M855 round at 940 m/s from a 1:7" barrel spins at about 316,000 revolutions per minute. SG = 1.82, which is comfortably above the 1.3 minimum and indicates good stability.
Use the stability factor as a screening tool, not a guarantee, because bullet construction, air density, and muzzle velocity all affect the real result.
The usual mistakes are mixing bullet length and weight, entering the twist backwards, or assuming a stable result at one velocity will hold at every temperature and altitude.
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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.