I will admit to limited experience of shooting and stopping big game rifles on big grumpy animals. But I am not convinced that horse power is the be all and end all of knocking any animal off its feet and rendering it completely unconscious, which is after all what you want when stopping an animal from either charging you or running off into the deepest darkest bush and depriving you of its carcass.

If a 100gn 243 bullet with a bit under 2,000 ft lbs is perfectly good for a 50 or a 100kg animal, then in proportion for a 1,000kg buffalo or a 7,000 kg elephant you need a lot bigger and a lot more horsepower than a 500gn bullet with 5,000 ft lbs.

Surely whats required for a good stopping rifle is a rifle that the hunter shoots and can hit a small target instinctively, and that then has a bullet with stout enough construction and enough energy and shockwave to cause catastrophic damage to the central nervous system so the animal collapses and the same time doing enough damage to major blood vessels so that it suffers catastrophic loss of blood to it’s major organs that it never regains consciousness. And a good knowledge of an animals anatomy helps as well. Provided the bullet has the penetration and is in the right spot it will work. I would question how much a bigger gun compensates for lack of good shot placement.

Certainly my experience on smaller animals is that a bullet from a 300 win mag in the guts will still go a long way. Whereas a 243 through the vitals will be dead on the spot.