I finished the campaign the last week and am now planning on achievement hunting (something I almost never do, however I feel compelled to do so!). Based on this wikia article it looks like some chapters are better suited than others for this task.
My question is: what exactly happens if I use the "Chapters" menu? If I remember correctly (not at home right now) it says "restart campaign at chapter X". What happens to your current saved game when you do this? For example, if I jump into chapter 4 then exit when I've finished an achievement can I still jump up to chapter 6?