The concept of "Stopping rifle"...
... is an interesting one...
stop / noun: a cessation of movement or operation (Oxford Languages dictionary).
From a kinetics perspective, Nothing short of a howitzer will stop in its tracks a charging Elephant (5 to 6 metric tons), Rhino (1 to 3 metric tons), Hippo (1+ metric ton) or Buffalo (1- metric ton), so the very concept of a shoulder-fired "stopping rifle" in the stricto sensu is somewhat flawed from a body shot perspective.
From a brain shot perspective. Most DG with both lungs collapsed and a exploded heart will happily transform you in an unsightly stain in the African dust, if they can catch you in the following minute or so. Therefore, as already stated in this thread, the only truly "stopping" shot is a brain shot. Period.
From this perspective, as also previously mentioned, any caliber/bullet that can reach the brain on a frontal shot is a "stopping caliber". Walter Dalrymple Maitland Bell had great success with the 6.5x54 Mannlicher and 7x57 Mauser. I doubt we would consider these "stopping" calibers, therefore there is more to the story...
From a non brain shot / missed brain shot perspective. This is where additional experiences are interesting. Another legendary ivory hunter (personal life-choices set aside), John Taylor, devoted considerable time and experimentations to the conceptualization of his knock-out factor, the Taylor KO. In his experience, dropping in its track a charging elephant could also be accomplished with knocking it out inconscient for a brief moment with a blow strong enough and close enough to the brain.
This is how, to make a long story short, and as already mentioned in this thread, the popular wisdom of shooting a minimum of .45 caliber / 500 gr bullet / 2150 fps / 5.000 ft. lbs. became a dogma.
I am not aware of too many modern folks having more experience than "Pondoro" and I see little rationale for questioning his conclusions, shared, it must be noted, by a comfortable majority of most any and all famed "White Hunters" of lore.
From this perspective, in Taylor's parlance, the baseline is around ~70 KO (.470 NE), and the .450/400 fails the test with ~50 KO. And so does even the full power .416 with ~60 KO. Those wanting a margin of error, and they were numerous, with John Hunter coming to mind, preferred the ~90 KO of the .500 NE.
Modern double rifles. Anything of 45+ caliber qualifies, assuming of course that we are discussing smokeless powder calibers, and the bigger you go the more KO you deliver. On both ends mind you, so there is a definite diminishing return for most of us mere mortals with such canons as the .577 (~120+ KO). Anything in the .400 range (~50 KO) falls below the threshold.
Modern bolt rifles. Notwithstanding 60+ year old horror stories of clumped ball powder in compressed charges baked by the African sun, the modern loads for the .458 Win (~70 KO) do qualify, but barely, so the modern wisdom for bolt action rifles has become the .458 Lott, with a recent interesting revival of the .500 Jeffery and .505 Gibbs (~90+ KO). As previously mentioned, the 416 (~60 KO) falls below the threshold.
I am not sure that there is any modern development, including monolithic bullets, that lends itself to successfully challenging the established wisdom, although, indeed, many tried and still try to rationalize choices they had/have to make based on financial availability.
