The drawing schematically shows the motion of a statically stable but dynamically unstable 7.62 x 51 Nato bullet on the descending branch of a long range trajectory.
Dynamic instability, in this case more precisely precession instability, results from a flowfield change after the bullet has been retarded. The Magnus moment changes into a destabilizing moment and the slow mode oscillation turns out to become undamped.
The combination of static stability - the bullet even has excessive static stability and responds to the wind force by moving its nose into the direction of the overturning moment - and an undamped (increasing) slow mode oscillation results in the indicated coning motion.
