Some of the scratches in the diffuse are too intense right now. Scratches in metal should be more subtle in the diffuse and bumped up in specular. Places where the AO is hitting should have a little more grunge since those are where each part of the model is intersecting. For triangle count, your entire cannon looks like it has more triangles than the bigger parts of the model; smaller parts of the model can have less triangles since you won't see them that much. Also for future reference, your cylinder caps should be manually triangulated since the game engine will do it for you if you don't and it won't look pretty most of the time when they do it Hope this helps.
A colored specular map is basically a texture, when applied, that will tell your model to project how intense and what color of the specular highlights should be from light sources. So if you leave your specular map black and white, your model will only have white highlights and instead of colored ones.
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A few crits,
I think the spec might be a bit too bright. I would suggest toning it down and upping the contrast a little to make the details pop a little more.
From first glance at the UV's it looks like you are wasting a little bit of space, especially with the legs.
I also think the body could sit up a little higher. Same with the gun. If it sat up a bit higher it would allow it too shoot higher and lower.