I'm trying to make a gun for an alien space ship and I need to know how to create a dark gray metal look. I have tried mixing a Diffuse and Glossy BSDF, but it isn't quite what I am looking for. I need some light falloff to help get that "extra touch", but I don't know how to do that.
I would recommend applying a normal map, using a glossy shader and then bumping up the roughness to your liking.
Here's what I did
Glossy shader color (hex)
When recreating real-world materials you need to have a good mental model of how they behave and interact with light from different angles. Collect as many images as possible of the material, preferably under different lighting conditions. This collection can be an ongoing library of images for future reference.
If looking for tutorials, don't limit yourself to Blender centric tutorials or even specifically ones for Cycles. Material shading techniques are largely generalized concepts that apply to any type of shading system -- which is great because you get the best of the Blender world and the best of the terrabytes of non Blender tutorials.
You likely need to look into Fresnel or the layer weight node. Also metal does not look like anything without something to reflect so add a HDRI light source. Last add a texture to your roughness. You might also want a good bump map.