My .300 win mag killed my 305 lb blackie at 70 or so yards instantly.Had lots of time to aim as the bear was just snacking at a large patch of raspberries.It Left a 6 or 7" exit hole broadside. Used factory Federal ammo with 180gr Hot-Cor bullets. What a mess. Heart wasn't even distinguishable.
Last year, I got a 340 lb blackie with a .30-06 and 180 gr Hornady Interlock over a load of Reloader 19. Anyhow, I again had ample time to aim, (this time at a little under 100 yards, and using a better rest.) The bear was hit well, but before I could recover from the recoil, the bear was gone????? I ended up not finding it until the next morning, and he was over 250 yards away, through THICK, almost impenetrable brush, and his lungs were almost liquified. Sometimes, a good bear just doesn't quite know that he's dead.
I just like the extra energy dump of the .300wm myself, but really, anything over a .30-06 or .308 is just overkill. I live in an area with the occasional Grizzly, I have seen one too unfortunately, so for me, the .300wm gives me more peace of mind than anything else. I don't carry it because I think its the minimum for black bears. Just use good bullets. I rarely use anything NOWADAYS other than GMX bullets, X-bullets, or really good proven bullets like the partition. (Or any good bonded bullets)
Bears aren't that tough to kill, but like any animal or human, an adrenalized version is just harder to kill QUICKLY.