Pressure Tank Gun

[Saintish]

It's way too easy to make, however. Maybe give it some sort of durability - the more robust the objects you fire, the faster the gun breaks. A full bullet would do 25 damage to the gun, whereas a spent bullet (assuming they don't do the same brute damage) would do 10-15. That's only an example, however; I have no idea how such a system would work in practice.