You will need PVC pipe, a rubber band, duct tape, string or fishing wire, and a steel ball for amo.
Step 1: Rubber Band and String
Tie the string or fishing wire to a broken rubber band.
Step 2: Tape
Drop string through the pipe a tape both ends of the rubber band to the pipe. Also put tape on the center of the rubber band to hold the steel ball.
Step 3: Grip
At the end of the string put flat pieces of tape over each other. This not only acts as something to grab onto, it also won't let the string go into the pipe.
Step 4: Shoot
To shoot simply load the ball into the pipe, pull back on the string, and let go. The gun shoots around an average of 10 feet.