IF you can find an airsoft gun (post a wanted thread on Craigslist) that'll be exactly what you're looking for.
Also, go to the army surplus store (New West or Broadway near the Skytrain station) and find some dud ammo. It'll look better if you mount 6 instead of a box, but a box will do in a pinch.
Also, you can do the same thing with a sawed-off shotgun (toy store, etc) and shotgun shells.
http://simplycomplex.us/dac/wp-conte...zombie_big.jpg
http://i.fosfor.se/i06/060819_2.jpg