This is an instructable which I saw a few years back but now seems inaccessible so please do not think I am trying to take the credit for the design. I'm just posting it in case the original is no longer accessible, hope everyone enjoys.
Step 1: Parts Needed
For this instructable you need two bulldog clips, an elastic band, a ruler, a pencil and some tape.
Step 2: Step 2
tape the pencil to the ruler
Step 3: The Ammo
Take the rubber band and attach it to the clip as shown.
Step 4: The Trigger
Attach the other clip to the other side of the ruler.
Step 5: Finishing Off
Loop the band over the protruding end of the pencil, pull the clip back and fit it inside the other clip.
And now simply aim and press down the clip which is attached to the ruler to fire.