I just got through reading this whole thread. Very entertaining!
A few thoughts on something I always find interesting...
I don't like it when people bring up a point like "well it might be good out to 100 yards, but what are you going to do when the trophy of a lifetime walks out at 400 yards?" This statement implies that all hunters are capable of adhering to a code of ethics - whatever that may be - UNLESS they are presented with a monster trophy, and then ethics goes out the window.
Of course, there are careless people in the world, but I believe most hunters do their very best to be as lethal and ethical as possible. I kill stacks of animals every year but I don't practice further than 200 yards. For this reason, I wouldn't shoot at an animal past this distance, be it with a .223, or a .300WM. The limitations for me at 400 yards are more about my ability to place the shot accurately, than they are about whatever the cartridge or projectile is, or what my rifles are capable of.
Thousands of medium-game (and some large game) animals are killed over here in Australia every year with a .223, or similar. Commercial shooters/harvesters typically use a .223, but some use .222, some use .243, and some use .22-250. I couldn't tell you how many pigs, fallow deer, and goats I've seen fall to a .224 centrefire rifle. The vast majority of my culling at night under the spotlight is with a .222 shooting 40gn Nosler BT's. Most of the shots I make are head shots but I've shot quite a few fallow deer through the ribs and they've all died fine.
Yes, culling at night from a vehicle with a good rest isn't really 'hunting', but for a long shot in the backcountry on a big target like an elk, I imagine you'll be using a decent rest of some sort?
The bottom line for me is that I usually use a larger rifle when I'm out and expecting to see larger game, but in our context over here, if you come across pest animals, they need to be destroyed, and I've had a .222 in my hands plenty when this has been the case and it's always done the job. I personally really like the 55gn Sierra SPBT in the .222. The slight drop in velocity compared to a .223 actually sees the bullet open up a little less, but penetrate further. Very mild to shoot and devastating on game.