Twist Rate Calculator
Use the twist rate calculator to estimate the bullet stability factor and the right twist rate for the bullet.
Required twist rate1 in 11.858 inchesGreenhill's formula with C = 150 for velocities below 2800 fps. A FASTER twist is a SMALLER number
Bullet length in calibres3.8961the real driver — Greenhill depends on the length-to-diameter ratio, so a long bullet needs a faster twist whatever its calibre
Constant used150derived in 1879 for black-powder velocities
Spin rate at the muzzle1.639e+5 rpmhundreds of thousands of rpm — far beyond any engine, and it is what holds the bullet point-first
Twist for a bullet half as long1 in 23.716 inchesa shorter bullet needs a SLOWER twist, which is why a barrel cut for heavy bullets over-stabilises light ones
Why specific gravity matterslighter needs fastercopper is less dense than lead, so a copper bullet of a given WEIGHT is longer than its lead equivalent and needs a faster twist. That is why rifles cut for traditional ammunition sometimes fail to stabilise modern lead-free loads
Greenhill's limitssolid lead, conventional shapesit takes no account of nose shape or boat tails. The Miller formula is the modern replacement and generally calls for a slightly faster twist
The formula
Greenhill T = C d² ÷ L × √(SG/10.9)
Length, not weight
Greenhill's formula depends on the bullet's length-to-diameter ratio, not on its mass. A long bullet needs a faster twist to stabilise gyroscopically, and since heavier bullets in a given calibre are usually longer, the rule that heavy bullets need fast twists is true only indirectly.
That distinction matters for modern monolithic copper bullets. Copper is less dense than lead, so a copper bullet of a given weight is LONGER than its lead equivalent and needs a faster twist — which is why rifles cut for traditional ammunition sometimes fail to stabilise lead-free loads.
Greenhill derived this in 1879 for solid lead projectiles at black-powder velocities, using a constant of 150. The amendment to 180 above 2800 fps and the specific-gravity correction came later. It takes no account of nose shape or boat tails, and the modern Miller formula generally calls for a slightly faster twist.