What does firearm caliber measure?

I assume it’s the size of the round; or maybe the bore. Is a .44 twice the size of a .22? And what is it 44/100ths of? Inches? Millimeters?

Fractions of an inch. So, a .44 is twice the diameter of a .22, and it’s .44 inches across.

Caliber really only tells a small part of the story. Bullet weight and shape along with the powder load tell a lot more. For example, there isn’t much difference in caliber between a .22 and a .243, but there is a huge difference in kinetic energy.

But it’s more complicated than that because barrels have two diameters – because of the rifling. Rifling is commonly .004" deep so the larger diameter is usually .008" greater than the smaller one. A 30 caliber barrel is .300" across the top of the rifling and .308" across the bottom of the rifling.

Also, there are many variations due to historical and marketing reasons. A .44 caliber barrel is actually .419/.427" across.

Also, calibers can be inches or millimeters. .38 is 38/100 of an inch, 9mm is 9 millimeter (obviously).

Unless you’re talking naval artillery. There, caliber is the length of the barrel divided by its diameter.

As well as the counter-intitivness that a .22 tends to be more powerful then a .25 or .32. And the fact that a 9mm, .38 and .380 are all around the same diameter, but are quite different, performance wise.

What’s even more confusing is how the cartridge names don’t reflect the true diameter of the bullet.

For instance, a .357 magnum has a .357 bullet. A .38 special also has a .357 inch bullet, and can be safely fired most firearms chambered for a .357 magnum.

Then, you have the 9mms. The 9x19 (also known as a 9mm Luger, Parabellum, or Nato) has a bullet diameter of .355, which is the same as the projectile found in a .380 Automatic round. It does not, however, have the same bullet diameter as a 9mm Makarov (.365 inches).

The .22s are the worst of all. First you have the .22 rimfire rounds, which all have a .223 bullet diameter except for the .22 mag, which has a .224 bullet diameter. For centerfires, you have the .222 Remington, .223 Remington, .22-250, .223 WSSM, and .22 PPC, all of which (and quite a few others) have an actual bullet diameter of .224.

Fun huh?

I’ve always thought that the 9mm is a .38 rechambered for an automatic handgun as a rimless cartrage. Am I mistaken?

Yes. :slight_smile:

Bullet diameter is only half (maybe one-quarter) the fun; case-length and chamber diameter also figure into it.

And to add to the mix, in naval rifles the caliber is the ratio of length of the barrell divided by the diameter of the bore. A 6"-50 caliber shoots a 6" shell through a barrel 25’ long.

It gets even worse. Typically in the US the caliber measures the groove diameter of the barrel which is typically the same diameter as the bullet. European calibers typically measure the smaller land diameter of the barre (lands are the bands between the rifling grooves. Many rifle cartridges use 7mm bullets. 7mm is equal to 0.2756" but the actual diameter of the bullet is nominally 0.284" Cartridges can be designated 7mm or .284 depending on the whims of who names the cartridge.

Sometimes the dimension will be fudged just to get a distinct name. The common .22 caliber is anything but. .22 bullets for centerfire cartridges come in diametes .222" (for the S&W .22 Jet) .223" (for .22 Hornet) .224" (by far the most common) and .227" There is the .218 Bee, .219 Zipper, .220 Swift, .221 Fireball, .222 Remington, .223 Reminton (same caliber as 5.56mm NATO used in M-16s) and .224 Weatherby just to name a few.

Oh, and .44 magnum/special caliber is not .44" Most have a nominal groove diameter of .429" AFAIK the designation is partly because of the heeled bullet design of the .44 Henry Rimfire. Even worse Colt designated his large caliber cap and ball pistols as .44 caliber starting with the 1847 even though the bore size is what everone else calls .45 to this day.

I thought it was the length of the barrel multiplied by the bore. :confused:

That is, I thought a 16-inch, 50-caliber gun was 16 inches x 50. (50 times the caliber of the gun, or 800 inches – 66-2/3 feet.) Although I’m a Navy brat, dad was Combat Aircrew and, after his commission, Communications Officer; so I don’t have any sort of background in Naval gunnery.

Clarification, please? :o

Somehow I get the queasy feeling that you are right. I’ve gotten division and multiplication mixed up since the 4th grade. But I’m working on it.

And to prove how screwed up I am (it’s early in the morning here) I just agreed that you were right when in fact you were wrong. Ah me.

You multiplied the diameter, 16" by the caliber and got the length. That’s the same thing as dividing the length by the diameter which was what I said in the first place.

I don’t need any help in being confused so lay off. :wink:

Heh. I was only half way through my first cuppa joe when I typed that, David. I’m only now starting to fire on all cylinders, but the choke is still out and I’m not running smoothly yet.

Ok, stupid question: when you buy a gun how do you know what types of bullets it will fire with all these variations?

You know what it shoots when you buy it. It should also have the caliber stamped on it. For example, “.45 ACP”, “7.62x51” or “.308 Winchester” (they’re the same round), “9mm Kurz”, etc.

…and ammunition is one item in which the specs are as close to universally accepted as it gets. You don’t just buy a “7.62mm rifle”, you buy a rifle that takes 7.62x51mm NATO or 7.62x39mm Kalashnikov or so on. The manufacturer will tell you what to feed it and what the metric/American conversion may be – in fact, even to the point of limiting what specific “loads” of that calibre you may use, as some rounds are loaded with a charge too powerful for your weapon’s construction.
Very few weapons will fire properly, or even function at all, if loaded with different-from-spec ammo, and the few that do accept only a minor variation – the most notable being a .357 Magnum revolver, which will easily fire .38 Special rounds because they are just shorter and with a lighter load than the .357 Mag, but otherwise fit the chamber exactly. With almost any other weapon, wrong ammo will either not work or damage the weapon, perhaps catastrophically.

The bullet is only the projectile. Many cartridges may use the same diameter bullet. Dozens of rifle cartridges use a .308 diameter bullet but are otherwise completly non-interchangable.

Bullets vary a lot in design. The correct bullet for .30 Carbine is fairly small at 110 grains and has a round nose. The .30 caliber bullet for a Winchester 30-30 cartridge must have a flat, soft lead nose. This is because the cartridges are loaded end end in tube magazine. A hard pointed bullet could cause the primer of the next cartridge to detonate in a chain reaction resulting in extreme unpleasantness. Some big game bullets are made for maxium energy transfer at impact at the expense of aerodynamics but most rifle bullets are spitzer shape with a curved nose that tapers to a sharp point. Target shooters will often use hollowpoint bullets but this is due to the manufacturing process. Putting the open end of the copper jacket at the front allows the base of the bullet to be formed more perfectly which makes for a truer flight.

Within a given caliber there will sometimes be allowable variations in bullet weight and design. For say, 9mm Parabellum bullets will range from 115 to 147 grains. A grain is 1/7000 of a pound. The lighter bullet will generally have higher velocity and lower felt recoil than the heavier one. They may be made completely of lead alloy (pure lead is rarely used in modern ammunition as it is too soft) or with a copper jacket and may have a round nose, a truncated cone with flat nose or hollowpoint shape.

You need to get Johnny L.A. and David Simmons to agree on the caliber of your floating gun ammo, and then let the invasion begin! :wink: