Very good idea for this comp
Started off by baking the shadows as usual, only I also added a plane underneath the gun, since in-game this will always be located on the ground and therefore the shadows will be more correct. Then I assigned different parts of the gun either dark gray or medium green and baked that out at 100% illumination as a "diffuse" to start with, to save myself the hassle of manually selecting and filling everything. From there I threw on a few overlays to take the diffuse from a flat color to more of a base. About half an hour after that I spent scratching the thing up manually. I really ought to make some brushes for that, but haven't. I guess it probably comes out better if done by hand anyway. After that came the screws in the main body and the holes in the barrel. I felt like it was lacking something, and, inspired by a show that was on in the background (Generation Kill), put the painted stuff in. There's a smiley face on top with some kill counters and a big old stripe through the middle. I figure, if it were to go in-game, these painted parts would change color to indicate which player owned the gun. Then I added rust around any jutting-out parts and called the diffuse done. The specular was done in the last 10 minutes, with the intention of making the dark metal parts very shiny and everything else a bit more matte.
If I had another hour, I would definitely have done a high poly bake, but I did not think it practical under the time restrictions.
Diffuse
Specular
