A spin-stabilized projectile is said to be gyroscopically stable, if, in the presence of a yaw angle, it responds to an external wind force F1 with the general motion of nutation and precession. In this case the longitudinal axis of the bullet moves into a direction perpendicular to the direction of the wind force.
It can be shown by a mathematical treatment that this condition is fulfilled, if the gyroscopic stability factor sg exceeds unity. This demand is called the gyroscopic stability condition
As the spin rate decreases more slowly than the velocity v, the gyroscopic stability factor, at least close to the muzzle, continuously increases. Thus, if a bullet is gyroscopically stable at the muzzle, it will be gyroscopically stable for the rest of its flight. The quantity sg also depends on the air density and this is the reason, why special attention has to be paid to guarantee gyroscopic stability at extreme cold weather conditions.
Bullet and gun designers usually prefer sg > 1.2...1.5, but it is also possible to introduce too much stabilization.
The gyroscopic stability is a necessary condition to guarantee a stable flight, but is by no means sufficient. Two other conditions - the conditions of dynamic stability and the tractability condition must be fulfilled.
