I'd like to play devils advocate.
For guard duty, something like a 45 would be perfect, especially since you're at a location that is considered a target for attack.
For just 'walking around', something smaller might be more appropriate. My logic for this is that the bigger the gun, the more likely you are to leave it behind when carrying it is a 'hassle'. For example, if you're wearing shorts and a t-shirt and out for a stroll, you might decide 'This is kinda big... I'd feel kinda awkward with this today, I'll leave it off' or "I just want to go jogging, I don't want this huge this bouncing around, I'll leave it here".
Something that you can carry casually, however, like a .22 (which, despite snorts around the BBS, is quite lethal. While it might not stop a charging truck, it should be more then adequete for 99% of thugs) might be better.
If I do a cost/benefit comparison, here's the chart I come up with:
.45
Pro
Stops anything/anyone
Con
Takes longer to aim
Weighs a lot, significant size
.22
Pro
Small, easy to conceal
Con
Not as much 'stopping power'
Now I look at the probable spread of who I'm protecting myself against. I've never seen the oft-described "300lb ex-football player, hopped up on PCP and charging" character that most heavy caliber folks cite. I'm not a police officer, and I don't intend to become a vigilante and go knock down doors at crack houses.
Next, it's very unlikely I'll ever need to use my gun in self protection, but I realize that and have decided that I'd rather 'have it and not need it then need it and not have it'.
Finally, I'm going to figure that, based on the above, in the unlikely event I DO need the pistol to protect myself, it's 95%+ certain that a .22 will be effective to the threat.
Weigh that against having the pistol on my person at all times instead of selectively, and I think the math shows that for me, a small caliber concealed pistol is more likely to save my life then a .45. It's not as powerful, but it's lethal and more likely to be with me when I need help, and that counts a lot in my book.
Police? Military? Guards? The equations work out very differently for them.
