Get some scratches and worn edges in there man, the mat is reading a bit bland because you dont have much of that. And also, throw a bit of a gradient over the entire thing, in the diffuse and the spec. Did you render out an ambocc map? Multiply that on your diffuse and spec as well.
Looks awesome man. Only thing I could suggest, is to add some gun metal stains and tarnish, to both the diffuse and spec. Even looking at that wikipedia image, you can see that it's got some green, red, brown and blue in the metal.
Been posting this in waywo, would appreciate some more in depth crits/suggestions. Wordy rambly version below image. I'm trying to round out my skillset a bit, to try and see about getting some contract work as a general or prop artist. I figured it'd be good for my portfolio to have a few examples of nextgen weapon…