What do you feel is the smallest .cal for self defense??

The penetration problem is an issue with no matter what you decide and Rob's advice is good and sound. It's all about comfort and accuracy. I have 3 weapons immediately at my disposal by the bed and which one I grab would depend on the scenario. If someone actually breaches to my bedroom door or I need to secure my outer perimeter, my 1911 with Tac light. If it's a front or back door breach, my Winchester Defender 12ga loaded with steel shot. If it's a middle of the living room breach my trusty Marlin .22 rifle with 10 rounds. Background, fire tunnels and enemy number's all play a role in my choice and it is automatic which one I reach for. Constantly train yourself mentally for all scenarios of what you would do and the problems you encounter and the solution will prevent itself to you and the answer to your question will be revealed. It doesn't really matter the calibre size, because I have seen a .25 drop a 255lbs man in mid stride and witnessed a man take six rounds from a .357 in the stomach and not only survive, but defeated his attacker with a clock radio.

Theres a saying in the industry, "Ametuers talk hardware. Proffesionals talk software."

Most of caliber choice is all opinion. If there was one wonder caliber that did everything, everyone would be using it. First and foremost, handguns suck at killing people. Dont believe that take a tour of detroit metro hospitol and see how many peopl survive pistol wounds.

Wound balistics are a complex field that has so many variable its not funny. Modern ballistics are so good that there is really only small diferences in between the 3 most popular defense calibers, 9mm, .40 and .45 acp.

What we start looking at is not only the balistics of the caliber but what it offers us in other aspects.
For me the 9mm is the best all around defense caliber and here why.

Round count: Round count is almost number one. We have seen time and time again .22 caliber pistols killing someone in one shot and someone taking 20 rounds of .40 and still trying to kill officers. Rare but it does happen. The point is we dont know how many rounds it may take to neutralize a threat. Taking on an EDP, intoxicated, or multiple threats at one time I really would feel more comfortable with my g17 and 15 rounds than my g36 with 7 rounds.

So, usually a 9mm will offer more rounds in the pistol. Hopefully also reducing the chance of having to reload, which takes you out of the fight for sometimes seconds at a time depending on skill level.

Recoil Managment: Anyone that actually knows defensive shooting will tell you the most important thing is to neutralize your threat ASAP. And you do that by putting as many holes in it as possible as quickly as possible. Now, YOU may be able to shoot a .40 cal faster than ME but YOU can put more rounds on target faster with a 9mm than YOU can with a .40 cal. Like I stated before in a multiple target situation you would want to shoot as fast as possible to gain more hits on multiple targets.

Cost and availabiltiy: Can you find the ammo to shoot and train like you should to be able to protect yourself and loved ones. 9mm is cheaper and easier to find than most other calibers there for i can train more with my money.

Like I said these are my personal opinions and should be treated as such. reguardless of what you have train with it and train often.

I feel comfortable with a 9mm but I do carry a .380 from time to time.

Here is a good true SD story that an aquaintence of mine had happen to him about 3 years ago...... He was at the atm at approx. 0215hrs in the morning when a suspicious man started towards him with a gun in hand ready to do whatever he had in his mind. My friend we will call him Tim for the purposes of this post drew a meager .22LR walther p22 and with one lucky shot above the BG's right eye down he went never to come back. Needless to say the BG died on the spot but it just goes to show you that yes shot placement is much much much more important than caliber hell if you are really good with a .17hmr then carry it im sure it has enough rounds to at least slow someone down enough to retreat which is the only goal of SD (to go home afterwords not to the morgue) I'm sure there are those people out there that will say the point is to kill him but what I was always taught was that the objective is to stop the immediate threat and if that means you have to kill them then so be it but trust me when I say going to extra mile after the BG is already down will def take your case from SD to Premeditated unless you have johnny cockran on your side LOL