This is the second incarnation of the design. The first was held together entirely with gorilla tape and I only had enough pictures for a slide show.
Though not as powerful as other designs, this thing can still do some damage. Do not point it toward the face, it could easily take out an eye. Do not aim anywhere that can potentially damage something, it's awkward aiming with the forearm, so its accuracy can be hit or miss (pun intended).
Step 1: You Will Need...
At least three Bic ballpoint pens
A thin rubber band*
Several craft (Popsicle) sticks
A length of strong thread*
Duct tape (No project is complete without it!)
A ring (optional)
*take care selecting these items. Too strong a rubber band can't be held back by the clothespin. Too strong a clothespin will break your thread. If everything is too strong you risk yanking the whole thing off your arm when you go to fire.
I was able to rig one of these with mostly scissors and a roll of gorilla tape in the back of class, so don't sweat the details. However, I have found these to work quite well:
A hot glue gun
A small-bit drill
A fine-tooth saw
Duct Tape (So important it deserves to be named twice.)