You need to build the pen gun I used. To help you with searching, I give you THIS. This is just an improvement of the pen gun

Step 1: Gathering the Materials

For this build, you will need an unsharpened pencil, some duct tape, a binder clip, and the pen gun.

Step 2: Attaching the Pen Gun

The pencil will be your base. Everything will be attached to it with duct tape. Anyways the first step is to tape the pen gun to the pencil as shown in the image.

Step 3: Attaching the Trigger

This is trickier than it looks, but possible. Tape the Binder clip to the opposite end of the pencil, on the same line as the pen gun. I find the easiest way is to flip the binder clip side that you are taping up, and taping it to the pencil. This is your trigger.

Step 4: How to Load

You load the pen gun by pushing the refill through the barrel, and placing it (and the rubber band) in the binder clip.

Step 5: How to Shoot

Flip the binder clip backward. You will shoot by pressing the end of the clip that my thumb is on. You will also see the pen refill in the clip. Press and FIRE!

Do NOT even point this at anybody and get me in trouble. The refill can slip through the clip and misfire. Also, even without the upgrade I made (Just the original pen gun) I got a lunch detention for shooting it in the ceiling of my school.