Greenhill Formula
An older rule of thumb, using a constant near 150, that estimates the rifling twist needed to stabilize a bullet of a given length.
The Greenhill formula is a nineteenth-century rule of thumb that estimates the twist rate a barrel needs to stabilize a bullet, working mainly from the projectile’s length relative to its diameter. In its classic form it divides a constant of about 150 by the bullet’s length in calibers to give the required twist, and the constant is sometimes raised toward 180 for higher velocities.
Greenhill’s rule is easy to apply with nothing but a length measurement, which kept it popular for generations, but it leans on length alone and ignores bullet weight and speed. For that reason it tends to lose accuracy with today’s long, light, low-drag bullets, and most precision shooters now prefer the Miller stability formula, which returns a direct stability factor rather than a single recommended rifling pitch.