We describe an electrical seismic gun that is capable of firing 8-gauge blank black powder shells in a water-filled borehole under relatively high hydrostatic pressures. The new seismic gun is a modified version of the electrical shotgun source for engineering seismic surveys introduced by Pullan and MacAulay (1987). Our modifications seal the firing circuit and 8-gauge shell against water entry so underwater detonation will occur reliably at depths to at least 80 m (0.9 MPa atmospheric pressure). Source energy is controlled by varying the size of the black powder load in the shell from 50 grains to 500 grains (10 kJ to 100 kJ). Although our seismic gun may be used in any seismic application suitable for modest explosive charges, it was initially developed as a versatile source for use in seismic investigations of the shallow subsurface (primarily engineering and groundwater studies). As of this writing, the gun has been used for optimum offset and CMP high-resolution seismic reflection profiling, engineering refraction surveys, fixed-source and variable-source noise tests, and vertical traveltime measurements in water wells. Other potential uses include VSP and borehole-to-surface or borehole-to-borehole seismic tomography.