Long time contributor to WWMM Carmen Salamone designed this crossbow quite some time ago and I've been wanting to make it ever since. Once I finally built mine, I wonder why it took me so long!
It fires mini marshmallows. And I mean fires them! We measured about 50 feet on average. The power is in the firing mechanism which uses Thera-Band...a type of super strong latex that stores and releases an enormous amount of energy.
Check locally: sporting goods retailers might carry it. Here's where you can buy Thera-Band online. You might also experiment with latex hose that you can find at hardware stores.
Here are plans for making your own. I've included a simple cutting template and a number of close-up photos to help you. The actual shape can be modified pretty much however you like. If you don't have a bandsaw, you can cut the pieces using a hand-held jigsaw or even a handsaw.
The only tricky part is attaching the stretchy Thera-Bands and rigging up the bow. Put the crossbow in a vise and just take your time tying it together.