Twist Rate Calculator

Your details

Find ideal twist: compute the twist your barrel needs. Check stability: evaluate how well a known barrel twist stabilizes this bullet.
The Miller Twist Rule is accurate for modern jacketed, boat-tail, and polymer-tipped bullets. The Greenhill formula suits blunt lead-core bullets and black-powder cartridges.
Total bullet weight. In imperial: grains (7000 gr = 1 lb). In metric: grams. Most .308 Win bullets run 150-200 gr.
gr
True diameter of the bullet body, not the cartridge name. E.g., .308 Win uses a .308 in (7.82 mm) bullet.
in
Overall bullet length from base to tip including any polymer tip. Longer bullets require a faster (lower number) twist.
in
Gyroscopic stability factor to design for. 1.5 is optimal for best accuracy and full BC. 1.3 is the practical minimum.
Muzzle velocity affects the Greenhill constant (150 vs 180) and the Miller velocity correction factor. Higher velocity slightly improves stability.
ft/s
Air temperature. Colder, denser air requires a slightly faster twist for the same stability. Standard is 59 F (15 C).
F
Elevation above sea level. Higher altitude means thinner air, which improves bullet stability - a given twist is effectively "faster" at altitude.
ft
Required twist (1-in-?)Marginal stability
12.8in

Minimum barrel twist to stabilize this bullet at the target Sg

Twist notation1:12.8
Atm. corrected twist (1-in-?)12.6in
Gyroscopic stability factor (Sg)1.5
Corrected Sg (atm.)1.46
Stability assessmentMarginal - minor BC penalty expected
BC penalty (approx.)3-5% BC penalty (marginal flight)
1.46 Sg
Unstable<1Poor1-1.3Marginal1.3-1.5Optimal1.5-2Over-stabilized2+

A 1:12.8" twist stabilizes this bullet at the target Sg.

  • Stability factor of 1.46 falls in the marginal zone (1.3-1.5). The bullet will fly point-forward but may incur a 3-5% BC penalty, affecting long-range trajectory.
  • A 1:12.8" twist means one complete bullet rotation per 12.8 inches of barrel travel. Lower numbers (e.g., 1:7) spin the bullet faster, while higher numbers (e.g., 1:14) spin it slower.
  • This bullet's length-to-diameter ratio is 4.03:1 (1.24 in / 0.308 in). Longer bullets (higher ratio) require faster twist rates to maintain gyroscopic stability.
  • Atmospheric corrections matter most at extreme altitude or cold weather. At 10,000 ft altitude, the effective stability factor improves by roughly 20% vs sea level with the same twist.

Next stepLook for factory barrels rated 1:12" or 1:11" to ensure adequate stabilization.

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