I'll go ahead and be the devil's advocate here and disagree with the notion that shot placement is "everything" and that with good shot placement the round you use won't matter. Good shot placement IS important. Absolutely. Practice, practice, practice, so that you can put the rounds where you want them to go under stress. That's not only important for stopping the BG, but also avoiding misses that could hurt someone else. Without accuracy, it doesn't really matter what round you're using. Conversely, WITH accuracy, I'll argue that the round you choose definitely CAN make a difference.
Now I'm not going to argue the tireless 9mm vs 45ACP vs 357 Mag issue. And I'm not suggesting that any round, to include a .22LR, isn't capable of stopping someone... eventually. I'm simply saying that even within a given caliber, some rounds are going to have a higher probability of being more effective against specific types of targets than others. So identify the target you have in mind (i.e. human BG), then factor in other likely variables (seasonal clothing, walls, or other impediments to penetration), then also consider other target variables that are way beyond your ability to predict ahead of time (mental state, intoxication, drug use). Ideally you want a single round that will effectively function across all the combinations of those variables. Obviously such a magic bullet doesn't exist, so we have a variety of compromising options from which to choose, based on the likely scenarios that we may encounter. Note that this might mean you choose a different round for your nightstand gun than you do for your carry gun, or that an LEO chooses for duty, even though all might be the same caliber.
I don't care how well you shoot on a range, I would challenge anyone under a stressful SD situation to shoot 1.5" groups. The CNS targets that basically count as "off" buttons are that tiny; the BG isn't likely to present a still, unflinching target; and the adrenaline dump you are certain to have will make careful aim nearly impossible.
So, to go along with shot placement, the crucial key to quick stopping power is proper penetration. Too little, you don't reach the vitals, no matter how accurate the shot, and therefore will fail to stop the threat quickly. Too much, and the bullet expends much of its energy outside the body, not to mention the risk to those beyond the BG.
The nature of the wound channel in this case plays only a minor role in quick stops, unless perhaps you do put the round directly into a critical organ, because unlike with a high velocity rifle round there isn't enough kinetic energy delivered from a handgun round for ballistic shock to have any lethal effect. Human flesh is highly elastic (save for a few organs like the liver) and will immediately seal over a wound channel, so you cannot count on bleed-out taking a BG out of the fight right away. The vast majority of gunfights are over in a matter of seconds, so you could empty 15 rounds into a BG and he still won't bleed out in that amount of time. To put a quick and decisive end to the threat, you've either got to hit something that counts and puts him down involuntarily, or rely on his physiological reaction to take himself out of the fight voluntarily. And with the deranged or chemically-altered state of mind that many BG's exhibit, you certainly can't count on the latter.
IMO, based on forensic reports I've read over the years, combined with simple physics/physiology knowledge and what to me just seems like logical reasoning (not to say those who disagree are illogical, just that my logic may be different than yours), the main factor is proper penetration through the target. That means not just through flesh or flesh-substitute like gelatin, but also through clothing, dense tissue, organs, and most importantly, bone. Bone is the #1 obstacle to reaching vital organs that are most likely to result in quick stoppage. I've read a forensic report of a 44 Magnum suicide (gun in mouth), with the round failing to penetrate the far side of the skull. Smaller calibers are widely known for skidding or deflecting off bone. If you read Jim Cirrilo's account of his first of many gunfights as a member of the NYPD Stakeout Unit, he made three headshots with his .38 special, and TWO OF THEM bounced or skidded off the skull without penetrating. There are few bones denser than the breastbone and skull, which by design happen to protect the organs that most of us train to shoot for.
It's been proven that heavier bullets better crush and break bone while continuing to penetrate, while lighter bullets are more prone to shatter or deflect off bone. If you've made that careful shot placement and actually put the bullet where you intended, you want it to continue on that path, and not deflect in a random direction! Otherwise your shot placement was worthless. Also, many people don't realize that if you have bone-crushing ability, your quick-stop target opportunities increase! For example the pelvis is a pretty large target -- shatter the pelvic bone and no human being can walk, no matter how strong or strung out. They simply won't have the structural support required to remain upright. Sure they might continue to drag themselves towards you, perhaps continue to fire if they have a gun (which means you shouldn't stop firing just because they've gone down), but you've instantly limited their ability to continue their attack.
So when I'm choosing a SD round within any particular caliber, I first look for the heaviest bullet. 147gr in 9mm, or 230gr in 45ACP, for example. If I were forced to choose a smaller caliber, I might actually choose ball since their penetration ability is going to be compromised already. Otherwise I rule out ball since it tends to over-penetrate and further reduce delivered energy. Now that I've narrowed it down to weight, I'll want to choose a bullet style that is more likely to remain intact rather than fragment and waste energy in different directions.
Therefore my choices are heavy bullets with 100% (or nearly so) bullet weight retention. Good candidates are HST and DPX bullets. Personally, I carry 147gr HST in 9mm and 230gr HST in .45ACP. I feel pretty certain that those will do the job AT LEAST as well as anything, and likely better than many other rounds on the market. If I expected the requirement to penetrate windows and car doors, like LEOs perhaps, I'd go with Gold Dots, since they have proven themselves as most effective in those situations. So bullet choice *should* vary depending on needs and expectations, but with the aforementioned criteria kept in mind.
Wow, I really ran off at the mouth there. Sorry for being so long-winded, I typed that out in starts & stops over a period of time, and didn't realize how much it was. :redface:
Just my (long-winded) $0.02... :wink: