For instance, the 5.56mm round (military version of the .223 Remington; .223 will "work" in a 5.56mm rifle, but a 5.56mm may not work in a .223
When it comes to calibers for self-defense pistols, the situations is at least as murky; the 9mm is probably the most common caliber for auto-loading pistols, but "gun people" will almost always recite like a mantra or holy writ that "any caliber is acceptable for self-defense. as long as it starts with a '4' and ends in a '4' or a '5'." Which is fine, as long as they are speaking for themselves, but is snobbery, at least, when it comes to making recommendations for others.
I often recite that myself when someone asks me what caliber pistol they should get, but then tell them that they
should not take it as gospel, but rather as my personal preference, and by the way, I happen to be packing a .38 Special revolver right now.
As for you, I suggest you try a variety of handgun makes, models, and calibers, to determine which will serve your purposes best. Generally speaking, the caliber should be the most powerful you can manage, i.e., accurate, safe, etc.When I was first certified as an NRA instructor, we generally advised people that the .380 ACP (AKA 9mm Kurz/Corto or 9mm x17) was the smallest caliber anyone should consider for self-defense. Then I ran into a lady who's self-defense piece was the Walther PPK her father had brought home from WWII. a .32 ACP pistol, it was small enough to carry concealed, she could shoot it well, and, of course, it was free.
And, of course, anyone who sneers at any caliber less than .44 Special will sneer at a .40 S&W, let alone 9mm or .38 Special. Snobbery, ignorance, prejudice... It used to be considered gospel among some that the .45 ACP was unmanageable; now it is considered the Gold Standard for auto-loading pistols. The 1911 used to be sneered at, now it is considered proof that John Moses Browning (pbuh) was the recipient of heavenly inspiration.
While it is sometimes found in "full size" pistols, the .380ACP comes into it's own as a backup or pocket pistol, when it can be carried under circumstances that preclude a larger carry piece. Keltec's P3AT is an excellent example of a reliable and easily concealed pistol; Smith & Wesson recently released their Bodyguard .380, indicating that this market has not yet been saturated.
So it would delight me to win the Lucky Gunner .380 ACP Ammunition Giveaway, as described at the Firearms Blog.
Lucky Gunner, BTW, came to the rescue last year when PayPal decided that they would not process purchases of raffle tickets for the Gun Bloggers Rendezvous, so they are doubly Good Guys.
***1. Although someone will invariably point out that there are documented cases of Grizzly and Kodiak Bears, and even elephants, being taken with a .22.
2. Grossly simplified to the point where I expect to be flooded with comments calling me an idiot and explaining the difference in technical terms. I'm trying to keep this accessible to, say, my mother, who isn't interested in that kind of detail, to say the least.