In the Breaking Bad finale when Walt turns on the automatic machine gun in his car's trunk everyone gets shot who is standing or sitting in a chair because the gun was pointed at a certain height. But how come Walt who is lying down on the ground gets shot too. Wasn't the gun way above Walt's level?
Maybe the its not the bullet as there is no bullet hole on his jacket and i am pretty sure the jacket is lying low. Maybe its the fragment that bounced of ground and hit near his appendix and further ruptured it. As taking bullet to the abdomen hurts bad and he can't really stay up that long.
And as @alessandro.francesconi pointed out the 10 number maybe relating the number of people died on the last episode (don't forget man on the outside and lydia). Just another theory.