There are some things you can always rely on, and a new Call of Duty game coming out each year is one of them. As one of the best-known names in video games, it's a series that needs little introduction. According to publisher Activision, it's sold an estimated 500 million copies, a movie adaptation is on the way, and despite having launched in 2003 it still reliably appears at - or near - the top of the annual bestseller charts. But this year the world's top military shooter might have a fight