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.)
Step 2: All Your Base...
What is important is that you make sure you use an odd number of craft sticks. While most of the sticks are flush, the middle one is pulled back about 1 1/8 inches. (all my measurements are arbitrary and based on how things fit together. Your measurements may vary.) This is where you will attach your clothespin.
Bind the sticks together tightly with tape. You may want to push it down onto your arm afterward to get it to contour to your wrist.
Step 3: The Pen Is Mightier...
Take one of the shafts and shave it down to a length that will fit sideways in your sleeve without making a noticeable bulge in the fabric. Mine came out to a little less than four inches.
Find the middle of your shortened shaft and make a hole all the way through. I was able to do this with an x-acto knife and screwdriver. If you have the tools, I'd expect a heated piece of metal to work pretty well. Make the hole wide enough for the nose piece to fit snugly inside.
Cut some notches in the ends of the shaft to hold your rubber band. Now attach the entire thing to the body of the crossbow with tape and/or any other fasteners/adhesives you are using, with the nose piece centered over the gap left from the uneven middle craft stick. Make sure the tapered end of the nose piece points outward.
Step 4: The Guide Rail
Take your clothespin and drill a hole through the two ends you pinch together. Position the clothespin on the body of the rig, over the craft stick that sticks out on the back end. Make sure the hole in the clothespin is far enough back so as not to be covered by the craft stick, and secure it down.
Take one half of the split shaft and shorten it to fit between the closed end of the clothespin and the nose piece centered between the crossbow arms. Glue it down.
Step 5: "Twang!"
1. Take a thin strip of duct tape and roll it up part way. Roll with the sticky side in.
2. Place one of your end caps on the edge of the unrolled end, on the sticky side. This part will hold the bolt (Ink cartridge) and keep it from falling off the rubber band. Place the rubber band just behind the end cap.
3. Cut partway in on the strip of tape on each side, behind the rubber band and before the roll. Fold these new tabs into the sticky area between them. Press what's left up the sides of the end cap. To be safe, you will want to secure the other side of the end cap with a shorter piece of tape with similar tabs cut into it. Do not make another roll with the second piece. This tape creation has to bear the tension of the rubber band, so try to make it as secure as possible.
Now time to "string your bow." The rubber band stretches around the arms, fitting into the notches you cut into the ends. The tape assembly should sit in the guide groove made from the half pen shaft. The rolled end should fit in the clothespin when pulled back.
If everything is in order, go ahead and tape on the other half of that pen shaft over it. Make sure to only use tape here as it attaches to moving parts and must flex accordingly. Anchor one end of the shaft to the top of the clothespin, and the other to the crossbow arms. You may have to glue in something to raise the top half of the groove up high enough that it doesn't slow down firing.
By now it's suitable as an office crossbow. Now on to the part that makes it unique...
Step 6: Finishing Up
Tie your thread to the outside arm of the clothespin before stringing it through the hole on the bottom side leading to the half shaft you just glued in. If your thread isn't strong enough to pull the two ends together you may want to double it up. After the clothespin, string the thread through the half shaft on the underside of the rig. An easy way of doing this is threading a sewing needle and dropping it through. Tie the open end to a ring, or into a loop.
Now it's time to add the straps. I made mine out of strips of tape, layered sticky-side in, fastened around my arm with more strips of tape. I made them kind of hastily to illustrate. If you have a good method to fasten the straps I would enjoy some feedback.
For ammunition, take the ink cartridges and cut them down to size. You want the back end to sit in the cup of the tape assembly when pulled back, and the point to fit closely to the nose piece without going in.
Step 7: Annoy the Heck Out of Your Friends and Coworkers
Make sure it is strapped to your inner arm if you want to hide it in your sleeve, that way gravity will pull it open allowing the your shot clearance. I would post pictures of it in my sleeve, but it hides so well it would just be a picture of my sleeve. What this contraption lacks in strength it makes up with in the whole "What the #&@% was that!?" factor.
For added strength, you can wind the free end of the rubber band around the nose piece.
Should the string break, you have a fail-safe. Push in the back of the clothespin with your free hand (You'll want to know where that is anyway, it's easy to bump it on something and shoot yourself in the hand).
Push the ink cartridge slightly into the nose piece (that's where it was made to fit). Now it won't fall out when you point your arm down. Make sure not to press it in too far, or when you go to shoot, it won't budge.
Add a safety by lengthening the string so it will only fire when the ring is on the second knuckle of your finger. Now when the ring is all the way down on your finger, you don't have to worry about accidentally shooting yourself in the chin when you try to pick your nose. When you want to get ready to fire just discreetly push the ring up to the middle of your finger.
Now just get creative! Add a place to hold extra bolts. Find a way to improve it (shouldn't be too hard). Personally, I've always wanted to see one of these rigged as a trap inside someone's desk drawer.