When i first saw this thread, I said "Oh, no, here we go again!", but i think most everyone here has posted wisely. The FBI spent a whole lot of money figuring out what went wrong after the big miami shoot out, and wound up adopting a 10mm loaded a little light, and later, a .40 Smith and wesson for the majority of thier agents. The key is accuracy, and penetration. I would feel better armed with a 22 and some deep penetrating loads, like cci velocitators, or the Aguila 60 grain SSS loads than I would any of the the shallow penetrating hollow point loads. The latter will penetrate about 16 inches in gel, and would likely disable a person by disrupting the spine or a large artery close to the back bone.
Personally, I carry a 44 magnum, it has big bullets that move very fast in pistol terms. I am a grewat big guy, and reall strong. I shoot a LOT, like 200 to 1000 rounds per week. Most people are mucj better off with a 9mm or 40 or 45, their choice, but they need to understand that if the bullet won't penetrate 12 inches or more, it might not get to anything vital in nature, even with perfect placement.
By the way, there is no such thing as "knock down power". Energy dump, as it is called in self defense circles, is a scam, and what's worse, a scam that will get you killed. As one gentleman posted, lookin at the "numbers" a 5.56 nato is just as powerful as a 7.62X 39 russian. In real life, as was noted, the 5.56 is at best a crippler, while the 7.62x 39 is a fight ender, and a widow maker. The difference? primarily penetration. The 5.56 does penetrate steel better than the other round, but in flesh and bone, the bigger round does the business.