Monte, a 223 will kill a PD at extreme ranges, IF you can hit it. There are those that are trying to shoot PD's at 2miles. How much retained energy/expansion will these give?
We are so lucky that we have access to all of these wonderful cartridges. Most are excessive for hunting but what the heck, variety is the spice of life. Over the years, we READ the exploits of many writers and use their experience as a basis for our gun choices.
The fact that many are funded by manf dollars is moot. New tech leads to new articles which sells magazines and rifles. Fashion changes our paradimes (sp).
In all this we have forgotten that HUNTERS have used the 303Brit and 7mm Mauser to kill all the big heavy stuff in Africa 100yrs ago. Today, articles are written that tell you need a fire breathing magnum to kill an 80lb whitetail from a tree stand inside 50yds.
The vast majority of modern hunters only get to take one large game a year, so in a lifetime, there is still very little experience. Throw in the normal screw ups of hunting and hunters want bigger power, just to be sure. At least that is what they have read.
So "rural" legends of so many ft-lbs, and premium bullets and magnum cartridges start. They make sense to most reading (lack of experience) and form a basis for comparison. Again, a situation of the marketer leading the inexperienced.
Not saying this is a bad thing, I like my RUM and 240gr MK at 3100fps, it can just lead to some conflicting conclusions. Why are there handgun hunters when a 30-30 is not enough? How come all those buffalo died when shot at long range with blackpowder cartridges - pretty "lousy" vel./energy stats?
My father in law has killed a lot of deer over his 35yrs of hunting. He doesn't know anything about ft-lbs, new cartridges, fancy bullets. He just knows that his 303 Lee Enfield drops them dead when the softpoint enters the boiler room. He also knows how to get to where he can make such a shot.
As always, use enough gun (so what the heck is enough gun) and get that bullet (any bullet)into the boiler room. Without a heart or lungs, any animal is going to expire. How long it takes is anyones guess, but eventually they will fall.
I think if you surveyed hunters anywhere in the world, you will find that there is no magic cartridge and bullet that will ensure one shot/drop'em in their tracks performance, everytime. Exclude spine, brain shots or 105mm rds.
The bottom line is that a skilled hunter armed with a "lethal" device, will take game at the ranges and situations he/she feels confident in. The rest of us need more practise...not necessarily bigger hardware.