Here is my take on it. it doesn't matter who you are or what you shoot, there are going to be occassions when your dead on aim at the heart is going to wind up missing and doing nothing but chipping off a piece of lung and tearing open some intestine.
now here's a simple fact. Smaller and lighter bullets cause less damage than bigger/heavier ones do. Slower bullets cause less damage than faster ones do. When the shooter fails to "do his part" for whaever reason, maybe even because the bullet was deflected by a bumblebee 100 yards out from the muzzle, if you have a wound that didn't hit real vital tissues, the one who shot with a .22 is going to lose the game, where the guy with the .577 NE will pick it up.
There are always going to be poor hits, and in all of those poor hits, using light calibers will result in a low quality wound, but a big cartridge will cause a more dangerous and more quickly lethal wound.
This is what defines a marginal round to me. Sure, if you put it through the lungs or heart, it will drop that animal. When God is on your side and promises that you will never miss the heart, I'm fine with you taking a 17 rimfire after moose.
But, for all the hunters who don't have absolute control of fate, you are never sure that your .243 will go right through the thing's heart without deflecting, breaking up, or just plain missing the vitals.
Some people are going to say that missing the vitals with a .243 is no different than missing the vitals with a .300 winchester, but come on.
Give me a break. A .243 will bleed out an elk or moose, PROBABLY. a hit in the same place with a 300 magnum, or even .270 will bleed it out quicker.
So, marginal? it is a cartridge that can be counted on to kill the intended game quickly and reliably when "you do your part," but if you bungle that shot, will possibly leave wounded game on the hoof that a more powerful weapon would have stopped much quicker.
Shooting with "marginal" weapons means that you have no room for error, and that a hunter is either ignorant of reality, overconfident in his skills, or just doesn't give half a poodlestick about the game.
This isn't like light tackle fishing, when a trout breaks off my 2 pound leader, he gets away with my fly. Big deal. When an elk walks away with a hole in his esophagus and no damaged arteries because that .22 hornet missed his arteries, its a crime against nature, IMO.
Once again, I'd like to point something out. Using a .243 to hunt elk instead of a .30-06 is in my opinion about the same as carrying a .32 ACP instead of a 9mm or .45. Unless there's a real good reason for carrying a .32, it's just plain foolish to do so.
The same goes for bear. Whenever bear defense is brought up, the discussion starts out with rounds capable of dropping a rhino, and work down from there, and the one sure thing to freak everyone out is to say that a .243 is good enough to defend against a bear.
I'd be an idiot to carry a .32 into east st louis, i'd be an idiot to take my .243 into black bear territory, but heck, carrying a .30 carbine after a bull elk, a .243 after moose, of for the love of god, a .22 lr after deer, is fine!?
Sorry, I really take issue with people who decide that it's better to go with light rounds than to go with heavier weaponry. It makes just as much sense as replacing your seatbelts with macrame.