Improvement of shooting accuracy of air gun pellets is important in sport competitions and demanded by shooting enthusiasts. Hence, the performance of a transonic spherical projectile as an air gun pellet with 4.5 mm caliber under a mechanism known as hop-up was numerically examined in the present study. Hop-up mechanism resulted in a rotational motion of spherical projectile, so a Magnus force was generated, which prevented the altitude loss of the projectile caused by its weight. The motion of the projectile was assumed in four degrees of freedom, including three translational motions and one transverse rotational motion. The projectile confronted the continuous variations of velocity due to the in uence of the aerodynamic forces; therefore, it experienced an unsteady flow. For numerical analysis of the problem, the 3-D compressible turbulent Navier-Stokes equations based on \\\\\\\\\\\\\\\\Roe\\\\\\\\\\\\\\\" scheme and dynamic equations of the projectile motion were solved in a coupled form as a fluid-structure interaction and in a moving computational grid. The results obtained from these studies showed that the proper rotation of the projectile, to a certain distance, could neutralize the altitude loss. It was also observed that the momentum of the projectile decreased by increasing its angular velocity.

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