Take a look at the example below. It's a simple homemade 12 gauge shotgun that only has two moving parts -- the trigger, and the bolt which is released when the trigger is pulled, slamming forward under force of a spring to fire the shell. Of course, if you don't have a gun it's likely you don't have shotgun shells either, in that case you could do away with the bolt and trigger entirely and make it into a hand cannon with no moving parts at all, using matchheads for gunpowder and a rocket igniter or a piece of steel wool with a 9V battery to set it off.
If you shot a bunch of airsoft pellets out of it though, even using a lot of gunpowder, it would be completely useless as a defensive or hunting weapon. You'd have to put something fairly heavy in it, like nails, rocks or fishing weights.
