When high-velocity penetrator impacts and penetrates a liquid-filled container such as an aircraft fuel tank, the hydrodynamic ram (HRAM) event occurs. This process could be roughly divided into four phases, each of which could cause different degrees of damage to the liquid-filled container or the surrounding equipment. Spherical fragment impacting tests of different velocities were performed on two sizes of liquid-filled containers to investigate the effect of boundary constraints on cavity growth. The velocity range in the experiment was from 600 m/s to 1400 m/s. Through theoretical analysis and experimental results, it is found that the radial disturbance range of the cavity is not constant in different containers and under different impact velocities. An improved method is presented to modeling the cavity growth in the drag-cavity phases of HRAM events. The approach quantitatively describes the radial disturbance range of the cavity and is appropriate for the calculation of the cavity growth in HRAM. Moreover, the effect of liquid type on cavity growth is studied theoretically. When the fragment velocity is less than Mach 0.5, the length and radius of the cavity are mainly affected by the density of the liquid. When the fragment velocity exceeds Mach 0.5, the characteristics of cavity shape are mainly affected by the acoustic velocity in the liquid.