I like this topic because it's brought up regarding bears a lot as well, and I live in bear country.
Is there a general widely accepted definition when it comes to "stopping rifle/caliber?"
Would it be wise to conclude that a stopping rifle/cartridge be a rifle that majorly, even with a missed brain shot (but within a margin, say an inch or maybe two?), will still end the charge or give plenty of time and safety for the follow up kill shot. This is also plenty of time for Average Joe, not movie star took 10,000 takes and is edited to be lightning fast magic type second shot.
I also wouldn't want to rely on the idea that you'll turn the animal, stopper, let's talk stopping.
John Taylor noted that a 600 NE with a missed brain shot may knock an elephant out for up to a half an hour.
Love him or hate him, Mark Sullivan, in his DVDs, generally running a 600 NE or a 577 NE has multiple charges where even he notes the brain was missed by an initial shot or two, and you can see it.
This is important, because when you watch, you see an animal that is still stopped/incapacitated even with a MISSED brain shot. This happens on Cape Buffalo and Hippo in his DVDs, including one cape buffalo where he blows the lower jaw off, something that looks like it would wake even the sleepiest coma patient.
It's also important because I believe he noted his first cape buffalo charge was with a 500NE, the first shot between the eyes missed brain just low and it was noted it had no effect on the buffalo, the second shot then hitting brain and shutting the buffalo down. So, 500 NE had no effect with the miss, where, in later videos we see the 600 or 577 do what needed to be done with a miss. So do we need 577/600 NE level numbers for buffalo...
The question then becomes what does it take to, a majority of the time, incapacitate said animal. Is it a ft/lbs number, is it 7,000+ on elephant, 5,000+ on buffalo etc...
Is it a combination, will bullet diameter really come into play here, for example, to knock out or the like, a buffalo, will it take 5,000+ ft/lbs in a 500+ caliber, but step to say, a 458, and now you need 6,000 +?
I'd cut gun and bullet construction out of this for this specific part. It has to be a given we're using quality here. I can't see expanding vs solid being a factor, unless one wants to argue the potential that a solid may pass through without dumping all energy, thus being less effective, perhaps, I suppose. Then expanding, but at what risk... IDK that this needs to be discussed yet.
I've got to think ft/lbs has got to be the big factor here, perhaps the only factor here, again, talking a head shot, missed brain but that was the aim, and given bullet is sufficient to penetrate skull.
How many here have shot a buffalo in the head, missed brain, but knocked that bull down still?
What were you using, bullet type, ft/lbs of energy? side of head, straight on?
Making a chart of instances like this would likely be the best way to determine this, what does it take on an elephant, a cape buffalo, a leopard, a brown bear, etc... one may be vastly different from the next and you may find the Australian water buffalo needing 7,000+ ft/lbs while the cape buffalo needs 5,000+, assuming because the animal is similar that like is needed may prove perilous.
Pro hunter vs. regular hunter becoming moot, again, remove variables that don't need to be there. What it takes to stop is what it takes to stop, the question of whether or not you can shoot it comes later and is personal enough to be about moot anyway. Also moot because, if it takes 600 NE or greater numbers but you can't shoot it, well, that doesn't magically make the 416 Rigby work, it just means you won't be using a stopper in this sense. Doesn't mean the 416 Rigby isn't good either, just means you better hit brain or you're relying on someone else to stop it.
Or do we want to define a "stopping" firearm/cartridge differently, I can't see why we would, to stop means to stop, it needs to stop the animal, not always, because life just isn't that simple, but majorly enough that you'd stake your life on it time and time again.
We know what Mark Sullivan had, he has stopped double digit numbers of charges at his feet, we know upon which he relied, 600 NE and 577NE. He did stake his life on it, time and time again.
The definition of stopper has to be defined here or we spin our tires.