I built a bb gun out of :
A bic eversharp mechanical pencil
two rubber bands,
a rubber washer.
1. Remove the slide insert thing from the pencil, and cut off the end. keep the shaft.
2. remove the pointy tip off the pencil. use a knife to shave out the end until a bb can fit into it.
3. slide the spring over the pencil shell until it pushes against the pocket clip.
4. put the washer on over the spring to keep it in place.
5. slide the insert thing into the pencil shell and loop the rubber bands around it, then loop them onto the spring.
6. load the bb from the muzzle end and pull back the eraser end of the pin. Aim at a non-living, non-moving target and release the eraser end.
7. It's not got a bad range for something cobbled together from junk.
Madd's notes: I'm working on an automatic, self-reloading model.