My theory about the gun and that opening scene is that Walt is already done by then and there's no real showdown coming. His cancer is back. Walt J. an Skylar are dead, possibly even Hank and Jesse. Walt has gone on the run. The final scene will be him taking the gun out of the back of the car and coldly opening fire on the headquarters of grey matter. Lol, ok not really since they showed the gretchen and the other guy live in ABQ, but it would be a huge difference to how people expect the show to end.