A convenient hand-sized version of Mr. Gattling's terrible weapon.
I made this out of a Nerf gun, copper pipe, rubber tubes, and lots of metal bits and paint. I painted the whole thing with a black primer that's specially made to bond with plastic, then masked off the different parts with tape and paper, so they could be spraypainted with different metal finishes, then detailed it with a greenish acrylic wash and some brighter copper, before gluing all the little things on. The glass tube on top has water in it, the idea being that you need to keep the steam chamber full of water or it blows up.
The glue I used was E6000 and Mighty Putty.