This project may not imply any of the wood and metal that are seen in traditional steampunk guns, but it is probably one of the cheapest way of making a toy gun look steampunked.
The creator of this project used a Nerf N-Strike Night Finder, but as far as we are concerned, you can really use any toy pistol that you want. Sniper rifles or even MP5s (provided that there are any toy guns that resemble the Heckler & Koch MP5) are good too, but you will have to adjust the rest of the materials used in the project accordingly.
Besides the Nerf N-Strike Night Finder, which is pictured above, in all its yellow and gray glory (by the way, does anyone else think it resembles the color scheme of a duck?), Cation Designs, the author of this project, also used some really inexpensive acrylic craft paints from Michael’s (Americana or Folk Art). More precisely, the color used were Raw Sienna for most part of the body of the gun, Pure Gold for the golden accents, Slate Grey and Metallic Silver for the parts that have a metallic aspect.
First of all, some rubbing alcohol should be used for cleaning the toy gun from any residues such as grease, dirt and dust. Next, you can proceed to painting the gun with the aforementioned colors or with ones that are very close to the presented hues. Depending on how the paint adheres to the plastic, you may need to apply two coats, to make sure that the underlying colors are not visible.
Personally, I think this would be a great way to turn air guns or paintball guns into steampunk weapons. It would be even better if manufacturers of air and paintball guns decided to come up with steampunk versions. However, this measure would also lead to premium prices for the guns, as the materials used are surely more expensive and very like more qualitative than the ones used in this project.
To cover the giant Nerf logo, that looks like anything but a steampunk element, Cation Designs used some buttons that have the steampunk brass aspect. Depending on the model of toy gun that you are going to use, this may or may not be a relevant advice.