I've thought of something, but haven't tried it yet. Dunno if it'd be safe to shoot with a pistol, and don't think you'd get the perfect reaction with a rifle since you'd have to walk so far back before you shot it, but I'll tell you guys and see what you think of it.
I dunno if any of you have heard about/seen the videos of what happens when you put Mentos in Diet Coke (or generic, cheap version of it), shake it up, open the cap just a little then throw it on the ground. It's like a cheap rocket, the pressure shoots the bottle up in the air. Videos everywhere about it if you search YouTube for diet coke and mentos.
So my idea is, put a whole roll of Mentos in a 2 liter bottle of Diet Coke, shake it up, then shoot it once. I'm not sure if the reaction will be violent and unsafe for pistol shooting, so I haven't tried it.