i am planing to make one actually. i was thinking to use a pvc pipe as the barrel. block one end. connect a copper pipe with a nozzle. and connect that to a closed metal container with water. heat the water till it gets pressurized then release. mine is designed to launch stuff so the pvc pipe will have a smaller exit hole and some sort of disc inside to launch smaller things. the drawing attached is crude because i didn't have much time but i hope you get what it means