I realize an animal and a human are different in terms of what it takes to stop them. However, our soldiers found out that the .223 is not nearly the stopper as a .308. An enemy shot with a .223 is likely to drag himself out of the line of fire before he croaks, where as the M14 round puts him down NOW. Both projectiles being hardball. Same thing applies to the military's switch to the 9mm from the .45. Now you might be thinking if the smaller calibers don't work as well, why don't they switch back? Because the little guns are easier for the girl soldiers to hit with, they can carry more rounds, and they can comply with NATO. All silly reasons IMO.
this argument has persisted since the introduction of the 5.56/.223 in the capacity of a COMBAT cartridge; namely in vietnam.
the military responded recently with the development of the 6.9 SPC for LONGER RANGE engagements.
if you notice, you'll see many of our troops carrying the in M14 rifle in Afghanistan and the USMC just announced a few days ago they are purchasing 12,000 new colt .45ACP pistols.
the military has finally awakened to the fact the 5.56 is 'adequate' within a 400 meter range...beyond that, it's more of a "Shoot and holler S&%#" proposition with the 5.56, especially when engaging hard targets. proof of that is that Gen Petraius himself, was shot in the chest with a 5.56, at point blank range, in a training accuident at Ft. Bragg...he survived to become head of the CIA...but that's not very good testimony for th 5.56, is it?
it's simple...bigger projectiles make for more damage/impact/KE/momentum and the faster you can move them, the more damage they are capable of. why do you think the 45-70 has more energy at approximaely 2/3 the speed of a .223?
tissue damage and penetration have to work hand-in-hand...broadhead arrows are a good example of that...i've shot arrows thru big animals that left wound channels and massive bleeding beyond belief...the arrow mass/weight forces the razor sharp broadhead thru tissue at no where near the power of a firearam...the broadhead severs arteries, major tissue and organs and, induces massive bleeding. a bullet must do the same.
caliber? it's directly related to the mission undertaken: the bigger the target, the bigger the caliber needed (speaking in a hunting scenerio)