I know something like this is kind of your job now, JSR, but that was pretty anticlimactic.
They at least could have loaded it with testicle-homing airsoft BB's...
EDIT: and dart_guy, there are airsoft grenades in the showcase section which were made (IIRC) around 2006-2007 by people with no machine tools, or even epoxy. Have a looksee.
EDIT 2: Can't find any of the diagrams PVC Arsenal posted back then, but basically they work by having a small chamber at a high pressure, with a larger chamber connected to a burst disk. To activate, a very small hole is uncovered which leaks air into the larger chamber until the disk bursts... pretty simple, but fiddly... I've never made one.
EDIT 3:
link