Barrel Twist Rate Calculator

The Greenhill formula scales required twist with the square of bullet diameter (T = C x d^2 / L), so diameter has roughly twice the proportional pull on the result that bullet length does. A larger-diameter bullet at the same length needs a slower twist (bigger 1:X number) to reach the same gyroscopic stability than a smaller-diameter bullet would.

A longer bullet needs a faster twist -- a smaller 1:X number -- because bullet length sits in the denominator of the Greenhill formula (T = C x d^2 / L). Longer bullets are inherently harder to stabilize gyroscopically, so they require more spin per inch of barrel travel to fly point-forward reliably.

Gyroscopic stability scales with a bullet's mass, and for a fixed length and diameter, mass scales directly with material density. A denser bullet -- lead, specific gravity 10.9, versus copper, roughly 8.9 -- therefore carries more angular momentum at any given spin rate, so it reaches adequate stability with a slower twist than a less dense bullet of the same physical dimensions would need.

C=150 applies to muzzle velocities up to roughly 2,800 feet per second, which covers the great majority of rifle cartridges; C=180 is the traditional adjustment for velocities above that threshold. Most hunting and target rifle loads fall under 2,800 fps, so C=150 (this calculator's primary Recommended Twist figure) is the applicable value for most shooters.