Well here's my first attempt at an Instructable so all constructive comments will be greatly received :)
I started with a AirBlasters Double Shot soft dart gun by Buzz Bee Toys. I was going to use a Nerf Barrel Break but the Double Shot just made a better base for victoriana / steampunk gun. The Double Shot also features auto-ejecting shells which are frankly amazing.
After disassembling the Double Shot (they are no where near as well engineered as the Nerf ones) I sprayed all the parts with a matt black spray paint. Just for fun (and to check I could actually remember how) I put the gun back together to see how it looked in flat black.
Now here's the bit I forgot to take pics of, but there is a combination of nut brown and brass paint for the stock and pewter spray paint for the barrel. After two layers in most places I attached the three valves.
The valves are stuck into three copper pipe brackets that I cut into so they would slot into each other. The whole lot were then mounted onto the side using a combination of screws and hot glue.
I also glued on a nasal spray bottle with a glass bottom. If I get time I might put a light in there as well.
Finally I added some brass coloured raised transfers (normally for cards and scrapbooks) to give it a bit of style and covered the finished product in some clear spray paint for protection.
Future changes / next model - I need to find a better way of attaching the valves, hot glue just doesn't cut it. Add a light / LED to the glass bottle of the nasal spray.