A sighting device is mountable to a gun. The device includes a light source (preferably a laser), a power source connectable to the light source and a mount attachable to the gun so that the laser is juxtaposed either the top surface of the gun or a side surface of the gun. In one embodiment, the sighting device includes a bottom rail mountable in a slot on the gun, wherein the slot is preferably positioned on the top surface of the gun. The sighting device may also include a mechanical sight that functions as the rear mechanical sight on the gun and/or a secondary light source.