I took a look at the skull last time I was in the Elephant museum in KNP and it's far too big to be a bullet hole. It looks to me like the hole was caused by a poachers trap which works by a tripwire triggering a weighted spike in a tree.

KNP claim that all of the 7 died of natural causes, but I have my doubts about that. Some of the skulls (in the museum) have a lot of bullet holes in them, some of which I would have expected to be fatal wounds.

If anyone's in the area the museum is really worth a visit.