I agree that 300 wsm is a good caliber. I have taken moose and bear with it.
The bear that shot 2 years ago was with 180 grain federal large game round, can't remember what the exact bullet was. I hit the bear in the right shoulder with a high scapula shot. A personal favourite, for a quick kill. So you are 100% correct the bullet quality and placement played a roll. I was also only shooting from 50 yards so that bullets was screaming at the time it hit an over 400 pound black bear. The bullet failed after passing through the scapula. Trailed blood for two days, until it was clear this bear was going to live. I hunt on my own land, so was lucky enough to to take this same bear this year. This time with 286 Barnes tsx in 9.3 x 62 at a sedate 2350 f/sec. Penitration was brilliant. Bear died in his tracks immediately. I very carefully dissected out the right shoulder and found my 30 caliber whole in the scapula and lead and copper fragments in the muscle and soft tissue between the scapula and the chest. There are very few times in your life that you get to confirm what happened when you screwed up hunting. This is one of those rare times. Bear skull is still frozen haven't taken it in yet, it would be pope and young, but I took him with a rifle, and not quite big enough for Boone and crocket. But most importantly, he tastes delicious, the experience of hunting him taught me a ton and made me a better hunter. Not to mention the cool story. That said, the pain of having wounded an animal, and presumed bullet failure, sent me on a massive search regarding terminal ballistics, bullet construction, sectional density and of course bullet placement. In Boddington's book he talks about black bears as almost being two different species the regular ones and the big ones. Knowing what, I know now- I wouldn't hunt big bears of any color with a fast light bullets at close range (180gn is light for big animals). And definitly not a bison.
But Med, I 100% admit, my own failings in putting together an inferior bullet, going to fast, at a big shoulder. I am very grateful for the lesson. I do think 300 wsm is a great caliber, and would suggest it is great for moose, small black bears <250 lbs, deer of all NA kinds, and probably perfect for long shots on sheep. Is best shot with high quality bullets, at distances over 100 yards. But that just my 2 cents after my adventures in bear hunting.
As to amazing shots with way to small a caliber, they definitly happen Karamjo Bell the most famous elephant hunter to ever walk the earth, favored using a .256 Mannlicher, and a .275 Rigby if you really know what you are doing you can use much smaller bullets. That said, most of us should just buy the right caliber, with the right bullet, and make good shots.
And yes, I would suggest Brett gets at least two guns, one for the big beasts, one for the small.