Both the U.S. Navy and the U.S. Army have considerable stockpiles of unguided munitions, such as the Navy's MK64 and the Army's MK483 155-mm (6.1-in.) spinning rounds. These rounds are relatively ineffective in the field because of their large miss distances, which result from a combination of gun pointing errors, shot-to-shot velocity variations, and aerodynamic disturbances that cause the flight path to deviate from the expected trajectory. As a consequence, the hit probability for a single projectile is very low. In fact, the single-shot hit probability for these unguided rounds is so low that over 350 rounds are required to achieve a hit probability of 90%.