Weta's Dr. Grordbort line of steampunk ray guns may be out of my price range (still dreaming of that lovely Pompson 6000), but these Nerf mods sold on Etsy look pretty great. The Goliathon, for example, is more than just you typical Nerf gun repainted with dry brushing. First of all, it's made from the venerable Nerf N-Strike Vulcan, a fully-automatic dart shooter that comes with its own tripod and ammunition belt. I had one of these on my desk at the Maximum PC office, which was handy for keeping interns in line. (This classic 500 RPM mod for the Vulcan is still impressive.) Esty maker Professor T. Lemetry also outfitted the Goliathon with metal parts, including gauges, copper piping, and brass accoutrements. Listed price: $350.
The mod appears to be a one-off and was just sold today. Hopefully its maker will decide take a stab at modding another steampunk Nerf gun--this time retaining its dart-firing capabilities.