No, a 7.62 won't necessarily drop a man with one shot- or even several. I once worked with a soldier who took 3 rounds in the chest (miraculously going through-and-through without hitting a major artery of the heart), who went back to full duty as an Infantryman after his wounds healed.
It's not the size of the projectile (unless you are using something REALLY BIG, like a .50 cal), but the amount of force that is transferred to the target, as well as the shot placement. One of the worst wounds I ever saw was inflicted during a domestic dispute with a single .22 Short round- but perfect shot placement, as well as some ricocheting around in the chest, abdomen, and pelvis. The amount of damage was incredible (fatal, in fact).
By the way, the article goes on to say that soldiers are trained to shoot "Center of mast". That must be the Navy he's talking about- in the Army we shoot "Center of MASS"! LOL! (You have to suppose the intrepid reporter got this quote by telephone, and knowing nothing about the subject, wrote what he thought he heard...)
Of course, I meant a "major artery OR the heart"! (The other way sounded kind of poetic, though, didn't it??)
You are probably losing around 200 fps which doesn't sound like all that much but since energy is affected very much by changes in velocity it might just be enough to make the .223 rounds ineffective or at least less effective.