I have very little experience playing with metal lighting/shading and guns in general.
However, on such a small piece the black outline is killing your ability to shape the piece. The curves look like straight boxy lines and the lighting is difficult to define that way.
I tried an edit. Took the contrast between your highlight colors and the black border down and cleaned up a lot of the color variation. Again this small you don't need 15 colors to define shades of grey. 3 or 4 is probably sufficient.
Mine came out really dark at 1x view, but maybe that's good for this size/distance.
Someone else will likely be able to help you much more than I can.