If it were frangible, it would have shattered and caused great damage, I think, the energy spent and dispersed locally by the action of the fragmentation. That is why I thought it was a metal jacketed bullet which could ricochet off his forehead if at an angle without penetrating the skull and hitting any serious matter. Perhaps it passed through something already before striking, so that its velocity was sharply curtailed.
Think of a rock skipped across the water or across a fat head.
Isn't the perp in possession of private property, a bullet properly belonging to the victim? How can he keep something not his? Unless he thinks of it as a gift...
Yes, it should have. There was damage; there are bone fragments in the wound. Perhaps it did shatter and that's only one piece of it lodged in the fat. Maybe the rest bounced off hard skull. Unless they extract it, who knows? Something is on the fritz here, and I was just looking for a theory. This one is pretty far-fetched, if only because frangible slugs are not easy to come by.