The vertical water-entry behavior of bullet-shaped projectiles was experimentally and theoretically studied. Particular attention was given to characterizing projectile dynamics, the resultant evolution of air cavity and particularly surface closure before deep closure in the moderate speed. We developed equations for the projectile motion with significant and negligible gravitational effects. Based on the solution to the Rayleigh-Besant problem, a theoretical model was developed to describe the evolution of the cavity shape, including the time evolution of the cavity on fixed locations and its location evolutions at fixed times. The gravitation effects during the initial stage of the impact of the projectile on the water can be ignored, but those during the later stage should be considered, many literatures do not have the report of this aspect. The theoretical predictions were consistent with the experimental observations. The evolution of air cavity had a significant effect on ballistic stability.