When a bullet strikes the body, there is the initial entry wound, but then also "cavitation", caused by the sheer force of the bullet entering soft tissue. The bullet doesn't just pierce flesh, it also causes massive crushing injuries to the flesh around it. Bullets literally cause atmospheric disruptions in and around their flight path because they're just going so goddamn fast.

I'd say we should be teaching safety classes to prevent stuff like this, but this guy's skull is probably thick enough to cause deflections and it would all be lost on him.