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?
I think when the bullets went through the wall their paths were deflected, so they weren't all flying straight anymore. Also Walt might have been safe if he'd been flat on the ground, but since he was on top of Jesse he was at a higher risk.
There is a little frame during the machine gun scene showing a "10-object" being hit by one of the bullets. In the very next frame, Walt has a jump while on the floor with Jessie. I believe he had been hit by such object.
Now the question is... why the number 10? :)
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.