Investigations of the underwater sound produced by shooting bullets into water indicated that the principal mechanism of sound production might be the oscillatory collapse of an air bubble near the surface. High‐speed motion pictures taken of a .38‐caliber‐pistol bullet's entry into a tank confirmed this idea. The main features of the acoustic signature can be explained and correlated with the motion pictures. On the order of 1% of the available kinetic energy was converted to acoustic energy. Most of the energy was transferred to a single bubble during the first 2 ft of travel in the water.