The Firing Line
The Greenhill Formula is commonly used to calculate rifling twist. Bullet weight does not directly enter into the equation. It assumes for a given caliber the heavier the bullet the longer it will be. The formula also assumes a specific gravity of a jacketed lead core bullet of 10.9. A series of equations may fine tune this equation, but generally:
twist for velocities under 1800 fps = 150 times D squared divided by length = twist
twist for velocities 1800 - 4000 = 180 times D squared divided by length = twist
My question is, do modern bullets that are not lead core(10.9 specific gravity) require a change in the formula?