Have been observing the effects of the 7mm RM, .300 WM and 338 WM for 30+ years on a lot of big game around the world. Have yet to see any vast difference in "killing power," given equal placement of a deep-penetrating bullet, put in the right place.
I suspect there are three reasons some people believe in a difference: 1) They believe the harder a rifle kicks, the "better" it kills. 2) They're more willing to put a bullet through heavy shoulder bones with bigger caliber bullets. In my experience, this affects "killing power" more than caliber or bullet weight. 3) As a result of 2, they firmly believe in "examples of one."
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.” John Steinbeck
