What "DVC" means and how it applies to you.
Accuracy of fire, the power of the rounds delivered on-target and speed of fire only come into their own if you're facing someone who is capable of seriously hurting you and actively trying to do it. If someone's not trying to hurt you, the amount of time it takes you to hit them is really irrelevant, because delay costs you nothing. We have words for those shootings: murder, or perhaps manslaughter. But it's absolutely not self-defense. Defensive shooting is a last-ditch solution to a fight you didn't start and can't avoid, where hesitation, failure or inaction is likely to result in imminent death to you or others around you. It's when things have to end right now and there are no options left but force.
At that point, it goes without saying only hits count. No doubt some people are deterred by gunfire in their general direction, whether they're hit or not, but it's folly to rely on that. The only response you can reasonably expect from an attacker is the one physically compelled. And only good hits have an expectation of compelling someone to stop--now. Center mass is always popular, not because it's most effective, but because it's the part of the body where incapacitation is possible--and you're most likely to be able to hit there under stress.
BEING GOOD ENOUGH
While not a guarantee of ending the fight, there's a tremendous advantage to getting off the first hit. Not only is it, as a friend of mine puts it, "hard to move fast when you're leaking," an initial hit can serve the same purpose as the first punch in a combination, like the famous "1-2-3" in boxing. It buys you time to deliver the next, more effective blow.
But none of that works if you miss. You must be accurate, and should never sacrifice acceptable accuracy for any other consideration. And I say "acceptable" because sometimes "good enough"--often is. As a general rule we're not talking about gilt-edged accuracy.
There's a lot of literature about the distance at which the average shooting takes place, and these things are good to know, but should not be relied upon exclusively. You don't get to pick the shooting you're involved in, or the distance. If you could, you'd choose not to be in one at all. The very fact you're acting in defense means you're responding to someone else's choices. While you can decide when to shoot, you can't control when they'll put you in the position of having to do just that.
This may occur at arm's length, on the ground after you've been sucker-punched from behind, or even from a great distance away. If you never practice beyond seven yards, then in the middle of getting shot at in a shopping mall by a maniac with a rifle is a really lousy time to discover that deficiency.
You should be capable of passable accuracy at most distances with a handgun, including shooting effectively and without hitting yourself at contact distance; with the understanding only reasonable accuracy at moderate distances is likely to be required.
Those who favor smaller calibers often say shot placement is what matters. It's just not all that matters. No pistol bullet can be expected to put down a determined opponent with a single shot. Something like 80 percent of people shot with handguns survive the experience. Understand the importance of making the most of what's available by picking the largest round you can, and train to maximize its effectiveness by putting those bullets in the best place you can, with multiple hits if necessary, as fast as you can.
The General's Advice
General Nathan Bedford Forrest was famously quoted as saying "Get there the firstest with the mostest," and that's pretty good advice. By the time a situation goes to guns, you're out of time. If you still had time, you'd be using it to leave. Once the gun comes out, expect the other guy to be constantly trying to kill you. Everything you do eating up time equals more time he has to shoot you. The only thing you can expect to stop the process is getting hits on-target before he does. The kind of speed we're talking about is the ability to deliver reasonably accurate hits faster than the other guy can shoot you.
Missing--even with a big gun--doesn't stop fights. Considering the relative unpredictability of a handgun in stopping a determined human being, the conservative approach is to select the most powerful weapon you can shoot accurately, at a reasonable rate of fire (including the initial presentation), then stay in practice.
Finding your own "DVC" combination is the trick. Don't be surprised if it's very different from your friends. That's okay.