I will show you how to create a pen gun out of the pen shown and a piece of paper. You can buy the pen a Wall Mart, Target, Staples, Office Depot, and tons of other places. This pen is VERY common. For the ammo you can shoot almost every thing that fits in the barrel although I prefer airsoft BBs.
Step 1: Know the pen parts
To split the halves just unscrew the pen in the middle.The first picture is a diagram of the pen parts. The second picture shows how to get the 2 parts at the back out by pushing it out with the ink. The third picture shows how to get the spring out by pushing the edge of the spring with a mechanical pencil with a inch of lead poking out.