Jim,
Finn Aagaard said only about a third of his Kenya clients who brought .300 magnums could shoot their rifle well, and a Montana outfitter friend says 20%. The Montana guy is mostly guiding pronghorn and mule deer hunters who think the .270 or .308 they use back home for whitetails isn't enough for trophy bucks Out West.
Another Montana outfitter I know who packs hunters into the Bob Marshall Wilderness carries a .375 H&H. I asked if it was for grizzly defense, and he said, "No, it's to finish off the elk my clients gut-shoot with their brand-new .338's."
