You have no idea what you are talking about, and you obviously have not seen the kind of damage a .223 can do inside a torso. Who is talking about headshots? The .223 is capable of explosive fragmentation. The 7.62x39 is not, and the .308 only very marginally. (There are complete physics models floating around that can accurately predict the terminal characteristics of cartridge/barrel combinations). For this reason, the .223 effectively delivers more energy and damage to the target than the 7.62x39, even though it has less at the muzzle. There is more mythology and anecdote than substance in the firearm world, unfortunately. Which is why there is a huge market for science-free voodoo in firearms.
There is a lot of big cartridge bravado, but not a lot of substance to back it up. Humans are not hard targets, and anything larger than a pistol bullet has sufficient penetration to do the job just fine. There is an old guy I know with more life-and-death trigger time than anyone else I've ever met, and on an endless range of weapons, who actually preferred the .30 carbine above all else over the decades (no accounting for taste). While I would find that to be a pretty questionable choice myself, he made the observation that the people he shot reacted pretty much the same way on average no matter what military firearm he was using, so cartridge stopped being a deciding factor in weapon selection. There is probably some good wisdom in there somewhere.
There is no man-portable Hammer of God weapon. You can find plenty of stories of people taking multiple .45 shots to the torso and head(!) without going down. I've seen enough bizarre failures to stop or penetrate with phallic-compensation uber-cartridges (and gobs with mediocre cartridges like the .308) that I understand the limited utility of anecdotes. Bullets are not a reliable way of killing people. As assault weapon cartridges go, the .223 is pretty damn good, though I suspect more by accident than design.
I don't know which ammo maker you work for but you lost me.
I spent 13 months in Iraq(03-04), 10 months in Bosnia(01), 1 year is Korea(90), time in Panama(98-99) and El Salvador(87).
In Iraq, I spent 16-18 days straight running convoys from Balad to Scania, through the heart of the Sunni Triangle, and through the now famous "Triangle of Death" south of Baghdad. As a platoon sergeant, I did everything from team leader, driver, SAW or .50cal gunner, mission leader, TOC operations etc.
I have enough stick time of the various weapons systems that we used, and I have read the reports of others in my platoon/company and what they dealt with.
You mention the DC sniper, then you say, "who is talking about headshots?" You mention "tumbling is a myth"; I was referring to the 'tumbling' that occurs inside the target once hit. You now reference that after stating that "tumbling" is a myth.
Everything I state is from my personal experience and the troops that served with me. None of this is theoretical for me. You can show me whatever ballistics reports you want, but when you hit Haji and he keeps shooting, and you shoot him again and again, then finally he dies and is no longer a threat, then that in my mind means the round is NOT effective.