Somethings wrong with that picture. The mans body is on the ground. The brain has very obviously sustained "significant" injury. It splattered out of the TOP of his head, and rests on the ground, directly OUT of his head. There's no gap in distance. Were he standing, and shot in the standing position, wouldn't there be less "linear" effect of the "effluent"? I'm almost betting that he was shot in the prone position, point blank. If that's the case, with muzzle on head, I could certainly understand a "large volume of gas" entering an "enclosed space", and then seeking the easiest means of exit.
Granted, I've not been "in the shit", so I really don't know what I'm talking about. But, I'm not believing that a 5.56mm projectile would cause such hydraulic shock. I will consult a man who's VERY knowledgeable about these things. Professor Whoopee.....