Sorry to be a bore [that's a firearms joke], but -- while I encourage everyone and their family to experiment with building such devices, with safety in mind of course, it is a lot of fun -- please don't dispose of your old batteries in the natural environment!

Really not trying to call you out here, as a teenager I once had the idea to load up my marble cannon with dead AAs (they're free!) and got a "do you really think that's a good idea?" from an environmentally conscious adult when I went asking for dead batteries.