Re: best bullet for nilgai in a 6.5x.284
My experience on a relatively un-hunted ranch was that the bulls were just plain spooky critters, and shots longer than 350 standing were the norm. I would get drops out to 500 for that caliber, practice a bunch, get a good rangefinder, and use the heaviest bullet available. They are tough animals - should be in the 450-600 pound range, sometimes a bit heavier. I would look at the toughest bullet that shot well out to 500 with that rig. Choices are not very wide right now but going to improve with the new bonded bullets that are coming.
Best right now might be the 140 grain bullet in a Nosler partition, a Barnes X or XLC, Swift A-Frame, whatever your rifle likes best. There are some nice 120 grain bullets in all of these but I would go with the heavier bullet for better penetration.
Nilgai hunt can be a really fun time.