Twist Rate Calculator — Rifle Barrel Twist (Greenhill)
Find the minimum barrel twist rate (1-in-X inches) needed to gyroscopically stabilise a bullet using the Greenhill formula. Enter the bullet diameter, length, muzzle velocity and core type to get the recommended twist and visualise how it changes with bullet length.
Bullet core type
in
in
fps
1 in 11.4" — faster (lower number) is also stable
- 1
Velocity-adjusted Greenhill constant C_adj = C × √(V ÷ 2800)
150 × √(2,800 ÷ 2800) = 150Scales the base constant for muzzle velocity relative to the 2 800 fps reference. - 2
Bullet diameter squared D²
0.308² = 0.094864 - 3
Minimum twist T = (C_adj × D²) ÷ L
150 × 0.094864 ÷ 1.25 = 11.38Result is the minimum barrel twist in inches per full turn.
How does this calculator work?
Greenhill formula: T (in/turn) = (C × D²) / L, where D is diameter (inches), L is bullet length (inches), C = 150 for lead-core or 180 for monolithic solid. At 2800 fps a 0.308-caliber bullet 1.25 inches long needs a 1-in-11 twist. Faster velocity → slightly less twist needed. Faster twist than the minimum is always safe.
Formula
How this is calculated
A bullet fired from a rifled barrel spins about its long axis; this gyroscopic spin stabilises it in flight, preventing it from tumbling. The required spin rate depends on the bullet's length-to-diameter ratio (its slenderness) — a long, slender projectile needs a faster twist (more turns per inch) to stay stable than a short, stubby one.
Sir Alfred George Greenhill derived his empirical formula in 1879 for elongated lead-core projectiles: twist T (in inches per turn) equals a constant C times the diameter squared divided by the length (all in inches). The constant C is 150 for standard jacketed lead-core bullets at approximately 2800 fps. Monolithic solid copper or brass bullets of the same length spin less easily due to their hollow geometry and different density distribution, so they need a faster twist — C is adjusted to 180. A velocity correction factor sqrt(V/2800) scales C for velocities other than the 2800-fps reference; faster bullets need slightly less twist because gyroscopic stability improves with rotational energy.
The result is the minimum recommended twist. Using a faster (tighter) twist is generally safe and can improve precision with long, heavy projectiles. An insufficient twist causes a bullet to yaw or tumble, producing keyholing (oval holes in the target) and severe accuracy loss. The Greenhill formula is a practical first estimate; more refined models (Miller stability formula, Berger formula) account for bullet mass distribution and air density but require additional bullet data.
Frequently asked questions
A "1 in 10" twist means the rifling causes the bullet to complete one full rotation every 10 inches of barrel travel. Lower numbers (e.g. 1-in-8) are faster twists, stabilising longer bullets; higher numbers (1-in-12) are slower and suit shorter projectiles.
Monolithic copper or brass bullets are longer than equivalent-weight lead-core bullets because the metal is less dense. Longer bullets need faster twists to stabilise. Additionally, their different moments of inertia change the gyroscopic dynamics, which is captured by the higher Greenhill constant C = 180.
Partially — the velocity correction factor adjusts for different muzzle velocities. However, as a bullet transitions through the transonic zone (roughly 1050–1340 fps) it can become unstable even with an adequate Greenhill twist. Subsonic bullet designs typically use heavier, shorter projectiles that are stable at low velocity with moderate twist rates.
Also known as
TG we-Calculate Editorial Team. (2026). Twist Rate Calculator — Rifle Barrel Twist (Greenhill) [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/twist-rate-calculator
TG we-Calculate Editorial Team. "Twist Rate Calculator — Rifle Barrel Twist (Greenhill)." TG we-Calculate. 2026. https://we-calculate.com/calculator/twist-rate-calculator.
TG we-Calculate Editorial Team, "Twist Rate Calculator — Rifle Barrel Twist (Greenhill)," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/twist-rate-calculator
@misc{wecalculate_twist_rate_calculator, title = {Twist Rate Calculator — Rifle Barrel Twist (Greenhill)}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/twist-rate-calculator}}, year = {2026}, note = {TG we-Calculate} }
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