XY, this is an interesting thread.
Is the 642 so powerful that one cannot provide quick follow up shots, no more than a half a second between shots, a third of a second would be even better, and hit an eight inch square?
Well, it depends on the shooter and the ammo (for speed of follow-ups), and the distance (for hitting the eight-inch square). Personally, I have no problem with quick follow-up shots using my 642 and +P ammo. It just hurts! And that limits my practice time considerably. By way of comparison, I can comfortably empty cylinder after cylinder of +P from my SP-101 until I get bored with it.
So what would be the best gun to have during an encounter, with the reality of stress effecting your mind, body and nervous system?
My opinion? A double-action revolver. More precisely, the double-action revolver you have with you at the time. I think any J-frame is a good choice if you carry it everywhere.
You've been practicing hard with your 642. Are you satisfied that it will serve you well if you really need it for critical shot placement or do you think that something less powerful like a .32 Magnum might be a better choice for easier control and marksmanship?
"Marksmanship" is too classy a word to describe the nightmare I hope to live through with the help of my 642. Like just about every other citizen who has had to shoot someone to save his or her life, I expect I will point my gun at the assailant rather than using the sights, and pull the trigger until the threat stops or my gun runs out of ammo. I also expect that, if it ever happens, it will be in the dark, within a few feet, and in a real big hurry.
I would say that if a 642 with +P is hard to hold onto when you shoot, then a .32 mag might be the better choice. You could also try non +P ammo in a 642, with lighter bullets, like the 110-grain Hydrashok. But if you're going to do that, you may as well go with a .32 mag and get the extra round.
I chose the 642 because the .38 round is available and fairly cheap, and because the better +P rounds are considered very good stoppers. The 642 also shares speedloaders with my SP-101, and I like to simplify things where I can. I also started reloading .38 recently. Eventually, I will work up a load that will allow me to practice more with my 642.