Not very hard to make and you can buy all parts in shop (or order) Ideas and pictures from http://www.officeguns.com
Guys I just want to apologise for copying everything from officeguns. I HAVE made one myself but I just haven't taken any pictures of it because I thought "They are all on the Internet, why should I take myself?" Sorry for that.
Step 1: Parts
1. broad rubber band.
2. 3 Mauly 32 (order them here: http://maul.de/php/Proxy.php?purl=/maul/products/catalogs/1000116/10/7/show,49.html
3. 4 Mauly 51 (larger than mauly 32) (order them here: http://maul.de/php/Proxy.php?purl=/maul/products/catalogs/1000116/10/7/show,51.html
4. Things to shoot with.