Effects on Ballistic

Spin drift

Even in complete calm air, with no sideways air movement at all, a bullet will experience a spin induced sideways component. For a right hand (clockwise) direction of rotation this component will always be to the right. This is because the bullet’s longitudinal axis and the direction of the velocity of the center of gravity (CG) deviate by a small angle, which is said to be the equilibrium yaw or the yaw of repose. For right-handed (clockwise) spin bullets, the bullet’s axis of symmetry generally points to the right and a little bit upward with respect to the direction of the velocity vector. As an effect of this small inclination, there is a continuous air stream, which tends to deflect the bullet to the right. Thus the occurrence of the yaw of repose is the reason for bullet drift to the right (for right-handed spin) or to the left (for left-handed spin). This means that the bullet is “skidding” sideways at any given moment, and thus experiencing a sideways component.

Discussion

  • Gabe Pettersen

    One other point I would include is that this only applies for spin stabalized objects. If the object was not spin stabalized there would be no yaw angle=repose angle and thereby cause the object to tumble because of the overturning moment arm that tends to rotate the bullet about it’s pependicular axis or velocity vector axis. However, the effect of spin does not completely nullify the overturning moment and therefore there is a slight movement up or “inclination” as you put it up for right handed (clockwise) rotating objects (opposite for left handed rotation=counter clockwise). This movement up is known as precessional movement or slow mode oscillation. Anyways, the best way to post/describe this is to post a bullet and assign appropriate vectors and moment arms. If I get time I will make and send you one. Take care,

    Gabe Pettersen, M.D.
    NorCal Practical Precision Rifle Club
    Sacramento Valley Shooting Center

  • http://longrangeshooter.com Sean

    Thank you