An easy to make office weapon capable of firing anything that will fit in the barrel.
Step 1: Gather Materials
All you need is a BIC mechanical pencil and two rubber bands. Pencils by BIC are perfectly suited to this (I haven't tried any other models yet). You also need a knife/razor to make an incision later.
Step 2: The Barrel
To make the barrel, you first need to get rid of the writing cone. The writing cone is the gray cone attached to the clear cylinder in the picture in step one. I find that the easiest way to do this is to let the cone hang off the edge of a desk, and break it off with your palm. Once the cone is off, you should be able to pull the eraser shaft out.
Step 3: Rubber Bands
To attach the rubber band, first put one end around the pocket clip on the barrel. Then, stretch the other end out to the opposite end of the barrel. Once you do that, you can use the second rubber band to tie it to the barrel.
Step 4: Firing Mechanism
Get the eraser shaft, and make an incision with your blade through the center of the eraser, coming from the top. You want to split it, not cut it off. Then, take the rubber band end off of the clip, and slide it into the notch you've made in the eraser.
Step 5: Completion
With the rubber band attached to the eraser shaft, slide it back into the barrel. The gun is now complete. To fire, pull back the eraser shaft, load ammunition into the muzzle, and release.
Step 6: Final Notes and Modification Suggestions
The gun is pretty powerful (but it's potential is defined by the type of bands you use. High-tension bands make the gun much more powerful.), but it does have it's shortcomings. Multiple impacts from the eraser shaft to the barrel chip away at the barrel, eventually making it useless. It's also got a lot of kick, it's pretty uncomfortable to fire if you're using really strong bands (even though some of the bands I used could put a BB through a soda can).
The way I described is the bare-bones assembly. There are many modifications that could increase the longevity of the gun and make it more powerful. On my gun, I took the clip off of the barrel, wrapped the entire barrel in electrician's tape, and proceeded from there. That reduces chipping from the firing mechanism. I also wrapped the eraser notch assembly with tape (just in case, and it makes the gun look more uniform). There is also a part of the eraser shaft that is not as wide as the rest, I cut it off and filed it down, so that the hammer takes up the whole width of the barrel, and leaves some space in the muzzle for ammunition even when the hammer isn't pulled back. Finally, at the firing end in the clear shaft, the opening isn't as wide. You can shave the excess plastic off to make for a uniform barrel width.