There are more than enough instructions out there on how to mod and paint Nerf guns for Steampunk. Instead here are some pictures of my guns to give you your own ideas. Both guns are functional and still shoot Nerf darts. After all, a functioning costume gun is always better!
Gun 1 is the original Nerf Secret Shot. It's hallmark was that it had a secret dart in the handle that could also be fired by switching the sight/dart holder down. I cut a hole on the top of the gun to move the "secret" launcher to the front. It might not be so secret now, but with some cut piping, it adds a bit more flare to the gun. On the other side I screwed some scrap metal to the side and screwed the read/write head from a broken computer hard drive to it. The metal piece on the handle is a Rare Earth magnet I took from a broken hard drive. It is for my Magnet gun holder Instructable here: http://www.instructables.com/id/Steampunk-and-Nerf-gun-quick-access-show-holder/
I eventually plan to wrap it in leather.
Since this was my first modded/painted gun, it didn't occur to me to paint the trigger or pressure arm until after I had put it all back together. Though, I might eventually do it later.
Gun 2 is everyone's favorite N-Strike Maverick. The stock gun already has a lot of relief markings that, when painted right, make it look pretty good as is. So the only modifications I did was remove the Nerf and N-Strike logo's from the gun. If you are going to do any Steampunk mod to a Nerf gun, you really should remove those logos! I left the words "MAVERICK REV-6" on the sides because I thought it still fit the look of the gun. And after all, Steampunk stuff had to come from something new in the first place. So my gun is just an unmodded new version of what other people might have!
Inside the handle of the gun I have placed a Rare Earth magnet from a broken hard drive. This is for my Magnetic Gun Holder instructable here: http://www.instructables.com/id/Steampunk-and-Nerf-gun-quick-access-show-holder/
My hope here is to give some ideas for others and show that care and thought in painting can make a big difference in Steampunk mods.