I have seen that photo from THE PERFECT SHOT many times. In fact, the guy who shot the leopard spent quite a while looking through the PH's copy before we'd even had a leopard coming to a bait.
One of the interesting things about minimum caliber regulations is how much they vary. Many hunters assume (apparently because they've "heard" it) that .375 is the minimum caliber for dangerous game throughout Africa, when many jurisdictions don't even have a minimum. Plus, in some places the minimum only applies to visiting hunters, not residents.
I know three very experienced PHs who've killed lots of Cape buffalo with the 7mm Remington Magnum, .30-06 Springfield and .300 Winchester Magnum--and not with head shots, but typical chest shots. The guy who used the .30-06 killed around 500 buffalo with handloaded 180-grain Partitions, with never a problem. The guy who used the .300 Winchester used 180-grain Barnes TSXs, same deal. The guy who used the 7mm Remington was really old-school, only insisting on 175-grain bullets of whatever type, again with no problems.
It's also interesting to compare minimum-cartridge regulations in various American states. Montana has never had a minimum for big game, or at least it hasn't since I started hunting in 1966, apparently because the game department assumed hunters know what works--though I have never run into anybody who uses the .22 Long Rifle on elk, ever though it's totally legal.
I will also note, however, that .22 centerfires are pretty popular not only for antelope and deer but elk, though the elk hunters I've known tend to prefer larger 22s like the .220 Swift and .22-250. One of my local buddies is a retired outfitter who used a 7mm Remington Magnum during those years, mostly to help finish off elk his clients shot in the wrong place.. But after retiring he switched to the .22-250, and has killed dozens of elk with it, using typical behind-the-shoulder rib shots with 55-grain softpoints.
Our neighboring state Wyoming, however, has been micro-regulating minimums for many years. Until 2012 the minimum for any big game was a centerfire cartridge of at least .24 caliber, with a cartridge at least two inches long. The next year they backed off a little, saying that .22 centerfires could be used on deer, antelope, mountain lions and wolves, as long as the bullet weighed at least 60 grains, and the cartridge was at least two inches long. But they retained the .24 caliber regulation for elk, moose, mountain goat, bighorn sheep and black bear--which is somewhat puzzling, since bighorns aren't any harder to kill than deer, and some people say they die easier (which is my limited experience).
All of this tends to make me suspect that most minimum-cartridge regulations are formulated primarily by a few people who had personal prejudices, rather than a wide consensus.
