Mirrors or metal doors. You can use any combination of colors to create the reflection; I just chose blue because I hate using regular gray for silver metal. The grad always starts from wherever the light would be hitting the surface. Any and every metal has a different reflection than another piece of metal. When highlighting and adding chrome streaks; remember the direction of light, the brightness of the light and the shape of the object that is being "chromed".
Gun barrels. Same technique as the leg, except this time, I don't have the Fade on. I use a larger brush then a smaller one, then I make the cut and airbrush through that until it's almost white in the middle. Then I get a color (blue, here) and put a little uplight along the bottom edge. This just makes it a little more reflective. Again, gun barrels aren't all shaped the same, but many times they can be round. Remember the shape of the barrel as you go through with highlights. This not only adds the chrome affect to the gun but it also adds the 3D reality.
Double-edged knives. Separate the knife at the middle and make the middle ridge the highest point. Do cuts like the mirror on both sides and put a bright cut down the middle ridge. Also be creative and add certain jagged or smooth techniques to the knife. The World is your oyster, make things how you think they should look. You want a knife with little indentations all over it? Then do that. Create the masterpiece that is in your head.
Chrome. This takes practice, especially on non-flat surfaces. I'll let you figure this one out on your own. (Hints: Be creative, find the lines that distinguish one dark shade from a bright highlight; look at chrome and metal objects around you and see what color they actually look like in bright or dim lighting. Study the chrome around you to get better, and never hesitate to create your own styles.)
Eventually though, as long as you know where you want highlights and shadows as well as what colors you what them to be, things start to get pieced together. You just need to think about where light would be hitting things and what kind of surface texture your working with.