Two of the most common areas of confusion for writers (and others) are the closely related ones of caliber and ammunition designations. These can be complicated matters, as will be seen, but the following should at least serve as an introductory guide.
With pistols or rifles, this may be expressed in one of two ways. Europeans give the bore diameter in millimeters and fractions thereof: 7mm., 9mm. etc. The American practice, and that of other countries using the English system, has been to express caliber in decimal points of an inch: .30, .38, .45 and so on.
Thus a .22 rifle or pistol bullet is, at least theoretically, twenty-two hundredths of an inch in diameter. When the Fifties paperback detective Shell Scott described a passing projectile as "forty-five hundredths of an inch across the fanny," he was quite correct.
That decimal point apparently represents an impenetrable mystery to some people. You'd think anybody would grasp the improbability of a ".9mm Luger" - a bullet only nine-tenths of a millimeter in diameter would be about the thickness of a good-sized needle; it certainly wouldn't inflict a very grievous wound - but even when the author doesn't commit this particular howler, he/she has to be careful because numb-nuts copyeditors and typesetters are liable to put it in.
Because of the international popularity of some calibers, different designations may refer to the same thing. James Bond's Walther PPK is referred to as a "7.65mm." but Americans call the same thing a ".32 automatic."
(Incidentally, the word "caliber" generally is added only with inch-fraction designations. That is, "a .38 caliber revolver" would be correct, but "a 9mm. caliber automatic", while not strictly wrong, would be contrary to standard usage. Everybody just says, "a 9mm. automatic" - and for that matter it's not necessary to say "caliber" at all; "a .38" is fine.)
Sometimes, however, especially with military weapons, the metric designation is universal. A 9mm. pistol or cartridge is a 9mm. on both sides of the Atlantic, if you're speaking of the common 9mm. Parabellum military round - what used to be called the "9mm. Luger" in this country. The less powerful 9mm. Short or Corto, however, is known in the US as the .380.
(Not to be confused with the ".380" revolver round issued by the British in World War II, which was simply the good old American .38 S&W. Somehow His Majesty's military had it in their heads that calibers must be expressed in three digits; they also designated the American .30 caliber rifles acquired through Lend-Lease as ".300". In this as in so many other areas, British usage is so weird as to be better ignored.)
Now it must be added that these "official" caliber designations may or may not accurately reflect the actual dimensions. In general the metric designations tend to be at least close to the actual measurements; but American and, to a lesser degree, British manufacturers have long had a tendency to round up egregiously and even assign seriously misleading numbers, perhaps for advertising purposes. For example, the .38 Special and the .357 Magnum fire bullets of the same diameter; the Magnum merely uses a heavier powder charge, with a slightly longer case. The correct measurement is approximately .357".
(By the way, "Magnum" should always be capitalized, whether referring to the firearm or to the dorky TV PI.)
Just to make the confusion complete, firearms - especially mass-produced military firearms - aren't always made to the most exacting tolerances. In some cases you might have considerable variation even among supposedly identical pieces. A "7.62mm." pistol - supposedly .30 caliber - might have an actual bore diameter anywhere from .308" to .312"; another one of the same make from the next year's production run might be a couple of thousandths different.
Not that these numbers mean anything in practical terms, except for people who reload their own ammunition and are trying for maximum accuracy; I'm just explaining that the official caliber designation isn't necessarily the exact bore size. But you needn't worry about any of this unless you've got some very specialized creative requirements.
Shotguns are classed according to gauge, which is determined by a weird and archaic system not worth explaining here. The only things you need to know are:
(1) The smaller the number, the larger the gauge - a 12-gauge shotgun is bigger and more powerful than a 20-gauge.
(2) There is no decimal point. A ".12-gauge shotgun", as you'll see mentioned in a lot of books, would be impossible for a man to lift, let alone fire.
Some popular cartridges for comparison purposes: .357 Magnum, .38 Special, 9mm. Parabellum (aka 9mm. Luger), and .380 (aka 9mm. Corto or Kurz). All these are exactly the same in diameter, yet they are quite different in their characteristics, and not interchangeable except that a .38 Special cartridge will fire in a .357 Magnum but not vice versa.
Modern firearms are loaded with cartridges. A cartridge is a self-contained little package which includes the bullet (unless it's a blank), a charge of explosive powder, and a primer, which is a kind of impact-sensitive cap that detonates when struck by the firing pin, setting off the main charge. All this is held together by a tubular case of thin metal, usually some alloy of brass although steel-cased ammunition is also encountered. (Particularly in military use.)
"Bullet" properly refers only to the projectile itself, and should not be used for "cartridge." However, this is quite a common misusage, even among people who should know better. An interesting reversal of a classic fallacy: thing contained for the container.
A cartridge may also be called a "round", especially in military usage. "He had six cartridges left" or "He had six rounds of ammunition left" are equally correct, and mean exactly the same thing. But it is not correct to say, as did one very popular female mystery author,"It held eight rounds of cartridges."
"Shell" properly refers to shotgun or artillery ammunition. All the same, it is common to hear "shell" or "shell casing" used to refer to empty cartridge cases, such as might be found at the scene of a shooting. "Brass" is another term sometimes heard, especially in the military or police; a detective at a shooting scene might say he was "looking for brass."
(Incidentally, these cases may be valuable evidence, since they can be matched by microscopic examination to the weapon, just as bullets can. If you want to use this, remember that they will only be found if the shooter was using a semi-automatic weapon; revolvers don't kick their empties out automatically - and it would be a pretty stupid perp who would empty the brass out of his revolver at the scene of the crime.)
Because of certain mechanical requirements, cartridges designed for autos will have different requirements from those meant for revolvers; and so a .32 auto and a .32 revolver, for example, will use different and non-interchangeable ammunition. However, some revolvers have been produced to use semi-auto cartridges, especially the 9mm. and the .45; but these are not very often encountered.
Some of these calibers and loadings exist on only one side of the fence. It is not uncommon to see references to a ".25 revolver", of which there is no such thing. Even God Almighty Himself, Mr. Elmore Leonard, made that mistake in his Western Escape From Five Shadows.
There is such a thing as a ".38 automatic" and indeed these used to be very popular; you'd be safe introducing one into a thirties crime story, for example. Nowadays they are pretty rare, having been largely supplanted by the closely similar 9mm. - same caliber, different cartridge, here we go again. In any case, the cartridge is entirely different from the .38 Special, which is a revolver round.
The ubiquitous .22 Long Rifle is used in a wide variety of revolvers, semi-automatics, and even derringers and single-shot target pistols, not to mention rifles. It is not terribly effective as a handgun cartridge, since it needs a rifle-length barrel to get up to full speed; still, a lot of people are killed by .22 pistols every year.
There may also be several different cartridges available in the same caliber, broken down by length. These will generally not be interchangeable in the case of autos, though revolver cartridges sometimes can be mixed and matched; and of course rifle cartridges are a whole different world. But this gets into some technical areas which should not come up in the average work of fiction. Or if they do come up, then you better plan on doing some serious homework.
Just a few members of the .30-caliber cartridge family. From left to right, the 7.62x54mm. round used in the Russian Mosin rifles and various machine guns; the 7.62x39mm. cartridge used by the SKS and the AK-47; the 7.62x25mm. ex-Soviet pistol round also used in the PPSh submachine gun; the .32 Magnum revolver cartridge; and the .32 S&W Long. All of these, despite enormous ballistic differences and applications, fire bullets of essentially similar diameters.
Numbers can be misleading. You might expect a .25 to be more powerful than a .22, but in fact the reverse is true - the .25 auto is such a pathetic little pissant that it is more likely to enrage than incapacitate the shootee. Indeed even the standard .32 auto aka 7.65mm., as in Agent 007's legendary Walther PPK, is no more effective than the common .22 Long Rifle round.
Caliber merely tells us the size of the projectile. It does not tell us the other part of the equation: how fast it will be moving.
As we all learned in high school physics, the energy represented by a moving object is the product of mass and velocity. Wild Bill Hickok's .36-caliber black-powder revolvers (similar to the modern .38) were much harder-hitting weapons than the larger-bore .41 derringer favored by his card-sharping contemporaries, because their bullets were moving faster.
Thus the .357 Magnum has the same bore size as the .38 Special - indeed some of the loads available use identical bullets - but its power is much greater, because its powder charge propels the bullets much faster.
As mentioned before, the .32 auto is a rather anemic little item, not taken generally seriously as a defense round, though admittedly a lot of people have been killed by them. Its bullet is very close in diameter to that of the 7.62x25 Soviet-bloc military round used by my Czech vz52, and indeed the bullets themselves are quite similar in weight and shape - but the 7.62 is a very powerful round, capable of punching through a bulletproof vest, and taken very seriously by the police of the world.
You can figure this up for yourself; if you have the bullet weight and velocity of a given round, you can calculate how many foot-pounds of energy it develops. Simple enough, so far; but hold on. There's more to this than meets the casual eye.
It might seem that if you jack up the velocity of a small-caliber round high enough, it will inevitably deliver more power than a bigger-bore weapon. On paper this is true; but people are not made of paper, and they don't die by the numbers.
The problem is that of effective power: how much damage the projectile actually delivers to the target. Which is a complex question involving such things as the shape and composition of the bullet and various medical factors.
And nobody knows the answer. A lot of people obviously think they do, but their conclusions differ so widely that it's hard to know which ones to believe.
For many generations it was taken for granted that a big slug, even moving at a fairly modest speed, would have more "stopping power" (the term is a bit misleading; it applies equally well even if the target isn't going anywhere) than a small one. Around the end of the 19th century, with the introduction of small-bore high-velocity weapons, it was for a time claimed that these would be equally or more effective, because on paper the energy equations looked impressive.
However, almost immediately the counter-argument arose: the zippy little projectiles, it was said, were too fast and too small - they would go right through a man without doing much serious damage. The common illustration was that of the icepick versus the ball bat: stab a man with an ice pick and he might eventually die of internal bleeding, but bust him with a Louisville Slugger and he'll go down immediately.
So the conventional wisdom remained largely unchanged; and so it remains today, on the whole. The general consensus among students of serious social shooting is that a combat handgun needs to be of fairly large caliber to get the job done: .40 to .45 preferably, with the 9mm. or .38 Special possibly allowed if powerful loads and expanding bullets are used.
And yet, and yet....
The founders of modern close-combat training were a pair of remarkable characters named William Fairbairn and Eric Sykes, who during the thirties served on the Shanghai police department at a time when that city was probably the toughest in the world. They took part in innumerable shootouts with formidable enemies (the Tongs or Triads were particularly dangerous to tangle with) and eventually developed a course of training that was later adopted by British and American special military forces.
They knew, in other words, just about as much about social shooting and its consequences as anybody ever has; and in their classic Shooting To Live they admit that they never did figure out the answer to the stopping-power question. They report cases of men being shot repeatedly with heavy .45 and .455 pistols and continuing to function (sometimes being subdued, in the end, by being hit over the head with the pistol that had shot them!) At the same time, they say that the most feared weapon in prewar Shanghai was the old 7.63mm. Mauser; its small, high-velocity bullets smashed bone and could inflict terrible wounds.
So there's no guaranteed-correct answer. Except, probably, the old truism: "It's not what you hit him with, but where you hit him."
Writers generally shouldn't have to worry about these details. Mostly it should be enough to know the approximate limitations of whatever firearms you've introduced in a story, and not go having your characters perform impossible feats with them - though, to be sure, that didn't stop Ian Fleming from making a hell of a lot of money....