This is easy enough to test at a shooting range. Start three feet from the target and fire three rounds as quickly as you can. If your range facility allows it, draw from concealment when shooting. Back up 10 feet and do it again, then repeat until you reach a shooting distance at which you are no longer satisfied with your hits. At this range you are beyond your own personal defense distance with that gun and you need to go forward to the last point where you got good hits. Are you satisfied with this distance? If so, you are done. If not, you either need to practice until you can get good hits at the maximum distance you think you may need to shoot, or you need to consider a different gun. This distance will be different for nearly everyone--it's a personal thing and something only you need to be happy and confident with. Jerry Miculek can hit a balloon at 200 yards with a J-frame Smith held upside down and fired with his little finger. All it takes is the time and money to shoot 6-8 hours a day, six or seven days a week...

As for caliber, the question about how much gun is enough for this or that purpose has probably been around since the second firearm caliber was invented. For the reasons stated in the previous post, I think the minimum caliber for semi-autos is 9mm P and .38 Special for revolvers. Personal preference kicks in beyond this level. If you have a personal preference for .44 Special and can conceal a revolver that big, fine and dandy. If you like .38 Super, .40 S&W, .45ACP or similar semi-auto caliber, go for it. If you are big enough and strong enough to shoot a magnum caliber accurately out of a gun you can conceal then one of them they may also work for you - but think about the over-penetration risks and how much farther away a missed shot might still be deadly because you are responsible for each bullet until it stops.

The number of rounds you will need is unknown. I've read that the average number fired seldom exceeds between three and five. Personal comfort must be your guide here - and for every other unknown involved. Keep in mind that if you choose a semi auto a spare magazine not only carries extra rounds but also allows you to compensate for a primary magazine's mechanical failure.

About these unknowns and personal choice options... If I am in a situation where I have to save my own life or that of someone else with a firearm, I am already in a wildly unlikely statistical situation and I see no point in expecting anything else to go my way. So, when I start making choices in preparation for this I'm probably going to err on the side of caution.

Should the ammo be heavy and slow or light and fast? I just pick a name-brand round designed and marketed for self defense and I set up to reload my practice ammo with the same weight bullet to hit at the same point of aim. This is another of those personal choice things--the "which is better" argument will never end.

Must a carry gun have a safety? For me, yes. Thanks to the list of assinine laws that criminalize carrying a self-defense weapon where you are liable to need it most, we have to unholster and reholster a gun way too often. Any time the carry gun is out of its holster and in motion there is a greater chance for Murphy's Law to raise its ugly head. Yes, training can minimize problems and the better trained you are the less likely you are to make a loud and unhappy mistake. BUT, the possibiility still exists because stuff happens and the more anyone handles their self defense handgun while doubled up in a car seat and fiddling with glove compartments or small safes, the more chances they have to screw up. I want a mechanical safety to prevent any kind of unintentional pressure on the trigger, from clothing, seatbelt hardware, keys, or anything else, making the gun go off.

What about forgetting to "off" a mechanical safety? You have to load a gun before it can fire, you have to align the sights and form a sight picture of some kind to make sure you hit your target, you have to apply pressure to the trigger properly to execute an accurate shot, and all these things are way more complicated than flipping a mechanical safety off during the draw. I don't recommend carrying 18 different types of guns with different manuals of arms, I recommend carrying one type of gun and becoming intimately familiar with every nuance involved it its operation. It's not like your life depends on it or anything, right?

Does a carry gun need a magazine disconnect? This reason for these, as far as I can tell, is to let you press the mag release button while fighting to keep an attacker from taking your firearm away from you and using it on you. The flip side is that you can't use it, either, unless you insert another magazine. This is another of those personal decisions you have to make - a magazine disconnect could save you or kill you. For police officers who carry their duty guns in plain sight where anyone could make a grab for it, I can see the value of having one. For a concealed carrier, not so much. My personal hope is that an attacker hears a concealed self defense gun before he sees it, but, stuff happens and you have to make your own decision on this.

For lurkers and others who may not know my personal choices, I carry the only kind of gun I have trained with and shot in bullseye and action pistol competition. I carry a full sized, lightweight 1911 in .45ACP, a gun and caliber I am very familiar with. I carry an 8-round Wilson ETM magazine in the gun and another as a spare. I carry the gun in a Milt Sparks Summer Special II holster. I prefer an inside the waistband holster because it conceals better than an outside the waistband holster, although my mode of unstylish dress would conceal either type. I wear the holster at about the 3:30 position on my belt. It is far enough back for an easy draw and to aid concealment but it doesn't press against the back of a chair or car seat. I shoot similar 1911s twice a week and I shoot the carry gun twice a month so I stay current and comfortable with it.

And, this may be more than you wanted to know...