I wonder if the slightly backed out primer indicates anything. Maybe pressure, over or not, rose quickly enough for the hull to hold on to the chamber wall without setting back against the face of the action. Maybe, since the primer doesn't appear near as distorted as the case head sounds, the pressure may have been falling when the headspace changed enough to let the primer back out.

I wonder if the shooter ever noticed proud primers on empties before. Does the other barrel show signs that some dimension change might be going on ahead of the chamber. Concentric rings showing, I wonder what might have been cleaned up that few inches into the barrel. Just interesting is all, thanks again Doc Drew.