There have been many thousands of magazine articles and even books written on caliber selection for defensive firearms. Shotguns of course always make great defensive firearms, but their large size makes them a distinct disadvantage in close-quarters combat. Since the legal limit on a shotgun barrel (without a special permit) is 18 inches, shotguns can’t be considered for concealed carry (although two companies currently make revolvers that can fire .410 shotshells as well as .45 Long Colt cartridges). The same goes for defensive carbines (rifles), which can be used for home defense, but can’t be easily concealed.
We are looking at handgun calibers. It is important to remember that any caliber choice will be a compromise. The overall goal should be to carry the largest caliber with the most stopping power that you can comfortably and accurately control. A .44 Magnum revolver will do you no good if you leave it at home in a desk drawer or can’t hit your target because of the recoil.
Conversely, on the other side of the spectrum, the diminutive .22 Long Rifle caliber remains the most popular caliber of all time by a wide margin, because it is supremely accurate, has almost no recoil and is inexpensive to shoot. In fact, it’s one of the best choices for beginning shooters because of these reasons. It’s a good choice to build your shooting skills. However, the .22 Long Rifle is not a good choice for personal defense (although it trumps most attackers armed with knife, baseball bat or anything else except a firearm).
The most important factor in caliber selection is stopping power. Evan Marshall and Edwin Sanow go into great detail in their definitive book by the same name, but basically, a bullet that both penetrates and expands to expend its full energy into the target (in self-defense, the human torso) is ideal.
The .45 ACP (Automatic Colt Pistol) has been a great choice for self-defense since it came along in 1911. The Colt 1911 pistol, designed by John Browning and used successfully by the military and civilians since World War I, is still the most copied and most popular of all American pistols, both for competition shooting and self-defense. There are at least 40 companies making 1911 clones. They are single-action, meaning that the external hammer must be cocked first to fire the first shot, and after that, the slide’s action will continue to cock the hammer with subsequent shots. The .45 ACP is a great choice, but the pistols in this caliber can be bulky and heavy to carry.
Two classic revolver calibers, the .357 Magnum and the .38 Special, remain great choices today. The .357 Magnum has more energy and velocity than the .38 Special, although the bullet diameter is the same (a .38 actually uses the same .357-inch bullet). But the case length is longer on the .357, so it holds more powder. Recoil is greater with the .357, so may not be ideal for smaller or more sensitive shooters. The .38 Special round (used by police for decades) will fit in and may be safely shot in a .357 Magnum revolver, but because of the casing difference, a .357 Magnum cartridge can’t be shot in a .38 Special only revolver.
The 9mm Luger (also known as the 9mm Parabellum) remains the most popular choice worldwide for police. With the development of the .40 Smith & Wesson cartridge, many law enforcement agencies in the United States have switched over from the 9mm Luger, because the .40 has generally more stopping power.
The .380 and the .32 are two popular semi-auto pistol calibers that are considered the bare minimum for defensive use. With the advent of advanced bullet designs (Speer, Hornady, Winchester, CCI, Remington and others have developed bullets that expand but also retain their weight and therefore provide good penetration), some of these smaller calibers can do the job, but may not incapacitate or stop an attacker as quickly as a larger caliber.
The .25 auto cartridge is considered generally ineffective, as it tends not to expand or “mushroom,” and the hole it makes is generally not much larger than its .22 Long Rifle cousin. But again, if you have a .25 auto with you and your assailant has a knife or something other than a gun, the pistol may prevail. Just don’t count on dropping your attacker quickly and have an escape route ready if you insist on carrying such a diminutive caliber.