A paintball team named Team Cynergy from New Jersey have created a laser activated paintball gun out a Red Dot laser sight. When the laser line is cut the Tippmann X7 paintball gun shoots. This would be a rather good project for building ready for a paintball battle as it would allow you to create traps around the arena to automatically attack the other team.
All the parts (except the gun and laser sight) can be purchased from a local electronics store.
Via: PaintBallGuns