You would probably have better luck with a servo pan & tilt sonar module like this:
http://www.pololu.com/product/1605 and a laser range finder to pinpoint the target. You program the arduino to translate the position of the pan & tilt assembly (which you have to build using RC servos) into a coordinate system. Your servos
0 position represents a reference point. When you command the servo to move, you increment/decriment a position variable to keep track of where the sonar is pointing. You have a separate pan & tilt (2 servos) assembly for the laser range finder but the range finder gives you range so you program the arduino not to fire until the range is close enough to hit the target. You display your
sonar and laser range finder positions on separate lcds.