Add in the fact a lot of them are shot from a vehicle at poor angles as it runs away. Definitely contributes to the reputation of being hard to put down. I have seen .270 with 140 Bergers work just fine.The toughness of Nilgai is largely related to the fact that their vitals are further forward under the shoulder like a lot of other old world game species. Americans tend to shoot them in the crease or behind, missing most of the vitals. You hear the exact same thing with oryx needing a 300 WM. But even a 300 WM doesn’t help with bad shot placement. I would be confident that a 223 with TMKs would kill a nilgai handily if you hit them in the right place. Although I also agree with @wind gypsy that the likelihood of a guide letting them use a 223 on a bull nilgai or even a cow is low.