Firearms and hunting legend Jack O’Connor wrote in his 1967 book, The Art of Hunting Big Game in North America:
One fall, hunting in British Columbia, Arizona and Sonora, I shot with the 130-grain Silvertip bullet of the .270 a moose, a grizzly, a caribou, a Stone ram, a desert ram, one whitetail deer, two mule deer, and one javelina. In all cases I considered the bullet action adequate.
The point I am trying to make is that the most important factor in killing power is the placement of the bullet. The second most important factor is bullet action—depth of penetration and rate of expansion. Compared to these two factors, bullet weight, bullet diameter, and even velocity don’t have much to do with the price of potatoes. The specific caliber is the least important of all.
Bullet technology has come a long way in the last 50 years, and still O’Connor’s advice is fine. While I’ll never kill as much game as he did, and while I can’t carry the man’s jockstrap when it comes to knowledge of firearms and ammo, I add this:
The old-school 130-grain bullet of the .270 that Jack loved and wrote about is still perhaps the best deer medicine of all time. I don't hunt with the .270 much anymore (not sure why) though I ought to. Over the years I've shot a bunch of whitetails and muleys with the 130-grain .270, along with a wild ram, several caribou and a couple of elk, though I never had the nerve to go for grizzly with it.
The 130-grain .270 just seems to have the perfect blend of weight, velocity and penetration that smacks bucks down fast and hard—if your bullet placement is good, on or just behind the shoulder. comment
PS: If you haven't read O'Connor, who was Shooting Editor of Outdoor LIfe for many years, you've missed out. The Art of Hunting Big Game in North America and The Hunting Rifle (1970) are my favorites.