On the second mission, The Island, there will be a man under the second bridge with a FG42 scoped. That gun will get more ammunition every time you kill someone. The gun starts with 96 ammo remaining and you can kill about 150 people with it (not including melee). After completing that mission you will have that gun on the third mission, but you must save the gun through the third mission to get it in the fourth mission and so on.