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.
Step 2: Assembly
2. Mount another Mauly 51 on the end of the Mauly 32 which isn't entirely covered by the first Mauly 51. Then put the third Mauly 32 on the end, as seen in the picture.
3. Remove the handles on the two Mauly 51's. Then flip the gun.
Step 3: Assembly 2
2. Mount the last Mauly 51 the same way as in the last step. Remove the handles on the two Mauly 51's. Make sure the position of the two Mauly 51's on top is offseted a bit. This makes the gun more stable and easier to fire.
Step 4: Final Step & Firing
Now insert a pen in the Mauly 32 and pull the rubber band. Release and it fires! WARNING! THIS THING IS VERY DANGEROUS!
You can also attach a laser pointer at top of the gun for better aiming.