Ok, this is not my project but I'm sure someone is planning this. Want to make your own airsoft grenade? Check out this video, maybe you'll change your mind. The most hilarious airsoft video I've seen so far.
http://airsoftcomics.com/index.php?option=com_content&view=article&id=81:why-it-is-not-recommended-to-make-your-own-airsoft-grenade&catid=48:video-guides&Itemid=58