Thank you for your service! ( I assume that's why you were in Afghanistan.)
You've eliminated user error and I assume you checked the sights to make sure they were tight.
I would definitely clean it out, Mossberg is bad for leaving a lot of shipping grease in their guns. (At least the shotguns, I assume the rifles are the same.)
I would hope that you will see a change with different ammo. I know in my '06 heavier bullets seem to fly better. I've tried a few times to shoot 150gr out of it and I can never get them to group as good as 180's regardless of brand. At one time I had two of them, same caliber and model. The other one would shoot the 150's better than the 180's. Some guns seem to just be pickier than others.