Coning motion of 
destabilized M80 Nato bullet

Coning motion of M80 bullet

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, or 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.

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