If you want to make gamesaves, you need to save just before the point of an achivement popping ( if you are making it for gamerscore) So for example if you want to make a save for getting 100 / 100 headshots , you will need to save at a point in the game when you have 99/100. Now when the save is resigned and used by someone else they will get one headshot and pop the cheevo for getting 100.
thanks bullet u beat me to it / i just wanted to see what he was asking 100% befor posting. dont like to tell people wrong things