It looks like a pretty good start.  As you said, the tires are horrible and should be replaced.   The M134 isn't accurate on an M923 "Hillbilly" gun truck in Iraq for a couple reasons.  These vehicles were only used for a short period of time before actual armor kits were added to them.   The type of mount and such on the gun wasn't used when these trucks were in use.  Also, the gun and its battery, ammo box, ect. would be too heavy for the truck's gun ring.  Lastly, M134s are not allocated to truck companies.  They are usually only used on special operations helicopters.  A .50 cal would be more appropriate.  Also, the water jugs you have in the back are a mix of different countries designs.  The one with the cap that has two pins on it is the US style.  They should all look be that type.  The US military doesn't use the other types.  They should all look like the one below in either sand or black.

Nice work on the weathered dusty look. And I love the rusty effects on the bare metal plates.

Gino, thanks. That's interesting, I'd have had no idea.