This will show you how to make a cheap airsoft gun out of a cap gun. This is not to be relied on in an actual airsoft battle. It's just a fun project to try.
7-31-09: Today I found that 12hippos posted the same project as me, but about 6 months earlier. Please note that I did not copy him, as I did not know the instructable existed. If you really want to see heres the link. http://www.instructables.com/id/SUPER_Cheap_Airsoft_Pistol/