This is a Nerf gun that I modded and painted.

(Yes, it still shoots!)

You can see another pictures of it here:
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and then the costume I made to wear with it is here:
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Basically I took some scraps I had laying around, including some of old costume jewelry, buttons, a wall socket and a wooden thimbly thing... and glued them onto the gun.
I glued with e6000 glue, but I used the hotglue gun to make a few accent bits around the socket with glue and to fill in a few parts.
(If you look for pictures of "nerf maverick" online you can see the original shape/style of the gun.)

I sanded off some of the "NERF" insignia (but not all).
I also took a knife to the gun and cut gouges into it to give it "battle damage".

Then I spraypainted it with automotive primer paint, and then spraypainted it with brown paint (which was the color base.)
Then I took gold paint and rubbed it on the gun with my fingers. This brought out the raised areas of the gun and made it look old.
I used a metallic aqua acrylic for the accent parts.
Then I sealed the gun.

The last step was adding the untarnished parts which are clock gears, a piece of dicrotic glass and a few more bits of costume jewelry.