Random screws or bits of metal (springs, gears, clocks, etc.) Anything you want. Even feathers or key charms.
Metallic paints. (Gold, bronze, copper, silver or whatever you want) I used Gold and silver for detail and copper for a 'rusted' look.
1. Use a knife to scratch lines and 'ware' into the gun. Also scraping away any numbers or 'made in china' text.
2. Glue or insert any bits of metal or screws into the gun where you please.
3. Using black paint (or black spray paint) cover the whole gun black.
4. After the black paint has tried get a sponge and rub the copper metallic paint into the tip. Then brush over random places on the gun to add a rusted and worn look. Try to keep these smudges as random as possible but focus on areas that would have the most ware such as the handle or protruding sides.
5. Add detail to the gun using the gold, silver etc. metallic paints. Use a small paint brush to do this. Paint chunks or keep it simple and only detail small portions of the gun.
6. The gun should pretty much be complete at this point. :D
7. Optional. Add your charms or extra bits of steampunky bits and pieces to the gun. I like my gun without any of this as its simple and clean. Again this is up to you and feel free to create a unique steampunk gun :D There are no set rules.