The Miller twist rule is better than the Greenhill rifling formula when measuring the specific gravity (SG) of a boat tail bullet because the Greenhill formula was originally developed for football shaped projectiles at subsonic speeds and is quite inaccurate for modern bullets. The Miller formula can calculate stability much more accurately for modern long range bullets. The Miller formula can also account for both velocity and atmospheric variables unlike the Greenhill rifling formula.

image of Miller twist rate formula

image of Greenhill rifling formula