Let's talk about popular handgun calibers and stuff like "stopping power." First off, let it be known that I wouldn't want to get shot with any caliber of bullet. Now that we're clear on that, let's enumerate some popular calibers and some of their characteristics.
- .22 caliber. The most readily available ammunition, even in times of shortage. Extremely popular for small game hunting (rabbit, squirrel) and "plinking," the term for casual target shooting. There are variants on .22, from sub-sonic rounds to .22 magnum, and depending on the firearm, the applications are varied. For self-defense, it's not terribly effective, but again - those little bullets can hurt or kill anyone, depending on placement.
- .25 caliber. Before the .380, one of the most popular "pocket pistol" calibers for very small and concealable firearms. Again, not my first choice for personal protection, because shot placement would be crucial. Like the .22, heavy clothing could seriously deter penetration of the round.
- .380, also known as 9mm Kurz (German for "short"). This has become a recently popular caliber, popularized by the many brands of personal protection firearms on the market, most of which are quite small and concealable. They're very popular with the CHL crowd. While the round doesn't pack the power of a 9mm, it IS a 9mm bullet, but with less bullet mass, and less powder behind it. Nonetheless, with some of the personal protection ammo available, it's an effective self-defense round. Most 380s are limited in magazine capacity however - it's a trade-off for concealment.
- 9mm. This is probably one of the most popular rounds, and there are so many variations on bullet weight, bullet type, and subsequent velocity and penetration that space prohibits detailing much. Nonetheless, a lot of police departments and our own military use the 9mm. There are those who disparage the 9mm, but it's an extremely effective round, and depending on the bullet can be a "man-stopper." You can fit a lot of 9mm into a standard double-stack magazine - 17 in a Glock 9mm, for example. You want more bullets in a self-defense situation.
- .40 caliber. My personal favorite. Lots of variety on the load and bullet type, good velocity, good penetration characteristics, good capacity. My competition pistol is a .40, and I can fit 20 rounds in my standard 140mm magazine. Not bad. Bullet weights for personal protection loads vary, usually using a lighter hollow-point (as with other calibers), which provides good penetration and expansion. Good balance of capacity and lethality.
- .45 ACP. One of the most popular calibers of all time, popularized by the invention of John Browning's 1911, the most celebrated of handguns. It's a very large bullet (230 grains, typically), but a bit slower than some of the smaller calibers. While many tout it as a "man" caliber, its lethality is contingent on a number of factors, including bullet weight and type, power, the length of the barrel, etc. It makes a really big hole, but you will sacrifice rounds (only 7 or 8 in the magazine) in exchange for a hoped-for "man-stopper."
- Anything bigger is just silly, or is only useful for hunting. Nobody seriously considers a .50 caliber pistol for anything other than hunting or just showing your friends. Not practical for concealed carry, self-protection, home-protection, or pretty much anything else.
As I indicated above, "stopping power" is contingent on a lot of factors, including bullet weight, bullet type, bullet speed, barrel length, distance of the shot, what it's hitting, etc. Again, movies would have you believe that a handgun can knock someone down, or even blow them through windows, shoot through anything, etc. Simple physics don't support that silliness. In fact, if you've ever seen anyone shot in real life (plenty of You Tube evidence), they typically jump, then run away. Depending on the shot, they will either live or die. If the placement of the shot is in the right place, the person may drop, but I won't get into that here.
I don't mean to treat the morbidity of "stopping power" in such a cavalier fashion - I realize how serious the topic is. But these are questions that I'm asked the most, and I'm just sharing what little bit I know about the topic. Got questions? Ask away.