.223 or .308
For quite some time, I had .308 and its equivalent [semi and bolt action], but no .223 rifles. While in the military, I did not care for the M-16, although I qualified expert and was diligent in the cleaning of my weapon. At a gun show in Richmond, I picked up a Bushmaster M17S bullpup at a really good cost (450 dollars - Spring '07). From there I bought a second M17S, an XM15-E2S, a Golani Sporter, a Daewoo AR-100, and a Mini-14 in bullpup configuration. Sprinkle in some other calibre purchases among the above listed. Of late I fire the XM15-E2S more than any other firearm. So much for not caring for the .223 round. If things were to get out of hand (yes, it could happen in America), I would grab my .45 pistol and the Daewoo. I would grab these two because they provide me with the greatest confidence to meet a civil threat. It comes down to your familiarity with the tool (in this case a weapon) and the confidence it gives you to use it (hopefully that day will never come). And be sure to practice [handling, loading, shooting and cleaning] otherwise on that day you will probably shoot a friend or yourself.
Stephen mentioned that we must sound like some really paranoid people to visitors on this forum (especially to the non-gun-wielding set). True, but this is our turf (as they have theirs), and the proper place to discuss such matters in an intelligent manner. I am sure Stephen did not mean any insult, but is concerned with our public image. At times we do needlessly hurt that image.