Yes, there are a lot of bullet sizes and it can be confusing. We call them “calibers,” except for shotguns, which are referred to as “gauge” or “bore.” Each is different and — just as with anything else — each has certain attributes that are beneficial for certain purposes…and its own drawbacks when the same caliber of cartridge is used for a different purpose.
How a round’s caliber is determined is usually from the diameter of the bullet (meaning the projectile at the end. That’s the thing that goes out the barrel when the gun goes bang or, in the case of metric rounds from one of them communist countries, the diameter of the bullet and the length of the case.
In days gone by, the measurement was taken from the diameter of the neck or base of the cartridge.
So, 9x19mm uses a 0.355-in (better knows as 9mm) diameter projectile (bullet), and the case is a hair over 19mm long. Ergo, the 9x19mm cartridge
The .38 Special, created back when ammo makers measured the neck, uses a .357-in diameter projectile but the cartridge case is .378 inches in diameter at the neck and base of the case. The .357 Magnum cartridge is named for the .357-in diameter projectile.
Rifle cartridges largely have the same conventions; metric or English measurement of the diameter of the projectile. Some, such as 7x57mm Mauser, use the European measurement of “bullet diameter” x “case length.”
Gauge or bore of a shotgun is measured in a wholly funkier manner. How those are measured is how many lead spheres the same diameter of the barrel would make 1 lb of lead. In the case of 12 gauge, twelve of them. In the case of 20, twenty and so on. The only exception is .410 bore, which is the diameter of a slug fired from a .410 bore shell. The shell is actually 0.455 inches in diameter, which makes it a .45 caliber shotgun.
So, now you can look at a cartridge name and get an idea as to how big it is. Let’s go over some common cartridge sizes and what they’re good for.
First is .22 Long Rifle. This is a .22-caliber rimfire round with a tiny case, chambered in pistols and rifles. This little pipsqueak round is good for target shooting, with almost no recoil and decent accuracy. It’s also good for small game hunting.
While it has been successfully used for self-defense (actually, more people die from .22LR wounds than any other caliber, but that’s just because way more people are shot with one than any other caliber) no one in their right mind would recommend you do. It’s also ridiculously cheap.
The smallest of the medium bore handgun rounds that most people consider for self-defense these days is .380 Auto, also known as .380 ACP and 7mm Browning. Other small calibers such as .25 ACP and .32 ACP have generally been considered too under-powered to stay popular. Not so with .380.
While .380 ACP is lacking in almost every category compared to 9mm, it offers just enough velocity and energy to be useful for self-defense. Its small dimensions and low recoil make it popular for carrying in micro pistols.
Now we come to 9mm, AKA 9mm Luger, 9mm Parabellum and 9x19mm. Velocity and muzzle energy (1000+ feet per second, 300+ ft-lbs) is sufficient for good penetration, good terminal performance with quality expanding ammunition, and the recoil is light enough for almost any shooter to tolerate it, even in subcompact rounds. It’s the standard caliber for NATO and most police department handguns for those reasons, namely a balance of power and ease of shooting.
While light on alleged stopping power, 9mm is considered the bare minimum for self-defense rounds. The FBI famously poo-pooed the 9mm due to failure to stop bad guys (such as in the 1986 Miami shootout) but it’s come a long way since then and modern ammunition has made it more than just adequate. It’s now the agency’s standard issue.
That said, the longest-serving personal defense and target round is .38 Special, devised by Smith & Wesson at the end of the 19th century. It’s a relative slow poke, rarely exceeding 900 feet per second and 300 ft-lbs of muzzle energy except for hot loads.
However, it’s highly accurate and laughably easy to shoot with a medium-frame revolver. Appropriate ammunition selection makes it very viable for self-defense, particularly when using hollow points. These attributes made it the default police round from the McKinley administration well into the 1990s.
The .38 Special was sometimes criticized for a relative lack of “stopping power” (for lack of a better term; there’s no such thing short of an elephant gun) and so Elmer Keith and a few other wildcatters tinkered with the load.
What they created was the .357 Magnum. A 158-grain semi-wadcutter bullet in .38 travels at a slow-ish 800ish fps. The .357 Magnum dials it up to over 1200 fps.
Good for target shooting, exceedingly effective at self-defense (a great many police officers carried it) and also decent for handgun hunting of small(er) game, it’s one of the great all-arounders. The .357 round, however, best shot from medium-frame revolvers, as recoil in a smaller gun is almost irredeemably punishing.
Another common handgun caliber is .45 ACP. Short for Automatic Colt Pistol, this round is the same bullet loaded in standard .45 Colt revolver rounds stuffed in a shorter, rimless case for use in semi-automatics. It was invented by John Moses Browning for the finest pistol known to man: Colt’s Model 1911.
The .45 ACP is very accurate and has decent-ish energy on impact. It’s great for target shooting and competition where the power factor matters and is a proven self-defense round. It isn’t shockingly powerful, though; it’s just big with plenty of bullet weight. Don’t be fooled by caliber junkies in the comments; it’s barely any more effective at self-defense than 9mm. However, if you insist on a big bullet, it’s the easiest of the big boy calibers to shoot.
Now onto rifles. Today’s rifle shooter is frequently a tactical junkie, even though they probably have an office job. Ergo, the more common calibers and bullet sizes of yesteryear (long-action rounds like .270 Winchester, 7mm Remington Magnum and the best cartridge yet invented by man, .30-06) are almost unknown to them.
Instead, the big deal is .223 Remington and its slightly hotter NATO counterpart, 5.56mm NATO. This is the standard round chambered in AR-15 pattern rifles. The .223 offers much more velocity than .22 LR (over 3,000 fps in some loadings) and more muzzle energy, but is still – make no mistake – a tabby among the tigers, so to speak.
The militaries of the world use it because it’s accurate and effective on hostile personnel out to 300 meters or so, but also because it’s cheap and easy to shoot.
In the civilian realm, it’s great for target shooting, varmint hunting and also dealing with small four-legged pests (coyotes, for instance) but is a piss-poor hunting round (at best) for anything larger. It’s effective for self-defense, but load selection is critical as over-penetration (bullet going through the bad guy, the wall behind them, and into your neighbor’s house) is a serious concern here.
Another very common rifle round is the 7.62x39mm. That’s the standard ammunition of the AK-47 platform. Some iconoclasts prefer the AK over the AR/M16.
The 7.62mm round is broadly equivalent to the .30-30 Winchester in terms of muzzle energy, trajectory and velocity, carrying more “oomph” than .223 but at the cost of slightly more recoil.
It’s decent for hunting at short range (just like .30-30) and you can shoot a lot of cheap surplus ammo for not a heck of a lot of money. It’s good for personal protection against two-legged critters; a whole lot of people are dead from being on the wrong end of an AK. However, load selection (expanding or frangible rounds are a must) is likewise critical.
The .308 Winchester – and its slightly less-powerful globalist counterpart, 7.62x51mm NATO – is currently the baseline for medium-bore, long-range rounds. It’s exceedingly accurate, and provides plenty of power for everything short of the great bears.
This round has longer legs, with flatter trajectory and plenty of retained energy out to about 600 yards for both hostile personnel and critters you want to put in your freezer.
While not as powerful as other .30-caliber rounds that use the same bullet size (.30-06, .300 Winchester Magnum) it’s still one of the best hunting rounds for North America and beyond for medium game. It’s also excellent for long-range target shooting, but with much more recoil than the previous two rounds. There’s also a lot of surplus ammo to shoot if you want to just burn through it…but the good stuff will start setting you back a bit.
So, those are some of the most common bullet sizes and calibers and the basics about them. There are hundreds more out there and plenty more to know, of course, but this is the Reader’s Digest version.