Mark Butler

A novice gun enthusiast walks into a store. The shop is bright and clean and smells faintly of wood oil. Hunting rifles adorn the walls and glass cases of handguns border three sides. Six or seven guys linger about, each sporting a bushy beard, combat boots and camouflage. The clerk behind the counter is seven-feet tall and his forearms are thick as tree-trunks. He frowns as the novice approaches.

“Sir,” the novice says, “I’m looking for an all-purpose hunting rifle. What do you recommend?”

The clerk licks his lips and indicates the selection, “Well, the Ruger 10-22 is a semi-auto rimfire, but we also have 17 HMR cartridges for slightly-longer range. A nice 30-06 is good for big game hunting, and if you want…”

“Excuse me,” another customer says, creeping up, “I couldn’t help but overhear your conversation. I recommend 7mm Remington Magnum, but the 280 improves accuracy! Now, the 300 Win Mag has…”

Another customer approaches. “None of you know squat! A 270 Winchester will get your job done, and if you’re worried about the government, an AR-15 will provide both…”

A fourth customer chimes in. “Ya need an ACR! You can get one in 5.56 or 450 Bushmaster, but I think…”

The novice gun enthusiast, seeing a fifth customer approach, slowly backs away from the group. The men are having a full-throated debate now. The novice continues to hear a strange series of numbers and letters that mean nothing to him.

“He needs a 241 LMT!”

“No, QRS 775 with 609 cartridges!”

“1000 ABCs you morons, 1000 ABCs!”

The novice looks around. The shiny pistols and rifles are no longer simple tools to serve a purpose. They are alien devices imbued with mathematical denotations that require 386 college-hours to comprehend. Seriously, where did these guys get this knowledge? Based on their passionate debate, they have engineering PhDs from Harvard.

The novice feels woefully uneducated. He doesn’t belong in this store. He knows little about firearms, but he watches the news, and he wants to be able to defend his family and property. Wiping a tear from his eye, he leaves the store, still able to hear the men arguing. He considers buying a knife, a can of pepper spray, a rape whistle – anything to avoid another gun-store experience.

That was an extreme example, but you get the point, right? The vocabulary that describes the firearms industry is a labyrinth of seemingly-random letters and numbers with various qualities and functions. This jumbled lexicon is not an accident – and it’s not by design, either. In truth, the confusing designations of calibers and firearms are the result of organic change.

New firearms and calibers are developed, old ones are discontinued. Some manufacturers maintain traditional naming conventions; others include extra information to help specify their brand. There are no hard-and-fast rules for caliber designations, and there is no single connoisseur who knows everything about every firearm. Regardless of age and experience, each of us is a student who can learn something new on any given day.

Now, let’s discuss the fundamentals of firearms and their calibers. We’ll review basic terminology, view a simple chart, discuss rimfire and centerfire, and conclude with some takeaway wisdom.

Some Common Terms

AR-15 – This stands for ArmaLite Rifle-15, not ‘assault rifle’. AR-15s are generally lightweight and semi-automatic and come in a variety of models

Blank – A round of ammunition loaded with gunpowder but no bullet – these are commonly used in movies, TV and military exercises

Bullet – The metal projectile portion of a cartridge that is propelled through a gun’s barrel

Caliber – the internal diameter or bore of a gun barrel

Cartridge – A type of firearm ammunition containing a bullet, propellant and an ignition device within a metal, paper or plastic case

Gauge – The bore size, or width of the barrel, for a shotgun.

Magazine – A metal container that holds cartridges under spring tension to be fed into a gun’s chamber

Recoil – The backward force of a gun when it’s discharged, also known as its ‘kick’

Safety – The mechanism on a firearm that prevents the trigger from being pulled

Silencer – A slang term for a suppressor. These devices attach to the end of a gun’s barrel and reduce the sound of its discharge

Now, let’s look at some actual caliber designations:

  • 7.62x39mm is a common caliber for many rifles. The ‘7.62’ refers to the diameter of the projectile. The ‘30mm’ refers to the length of the cartridge.
  • .30-06 is a common caliber for U.S. Army rifles, machine guns and hunting rifles. The ‘.30’ refers to the diameter of the projectile. The ‘-06’ refers to 1906, the year the cartridge was adopted by the mainstream firearm industry.
  • 9mm is a common caliber for pistols. The ‘9mm’ refers to the diameter of the projectile. Occasionally, this caliber is expressed as 9x19mm, whereas the ‘19mm’ refers to the length of the cartridge.
  • 20 gauge is a common caliber for shotguns. ‘Gauge’ refers to the weight of the cartridge, which in ’20 gauge’ literally means an equivalent weight to 20 lead balls that are the same diameter as the bore or barrel. By contrast, 12 gauge means each cartridge weighs the same as 12 lead balls that are the same diameter as the bore or barrel. Therefore, a cartridge for a 20 gauge has projectiles that are each 1/20 of a pound, whereas the cartridge for a 12 gauge each weight 1/12 of a pound. That is why a 10-gauge shotgun fires larger shots, and has more recoil, than a 12 gauge. A 12 gauge is larger than a 20 gauge, and so forth.
  • After a caliber’s numbers, there are often words or letters. For example, 9mm Luger, .40 S&W, .45 ACP, .38 Special, .357 Magnum. These letters generally represent a brand name or abbreviation, or they give more information about the ammunition. These letters are eminently important, and when purchasing ammunition, you should be familiar with exactly what type of ammunition you need and what the letters specifically mean!
  • A common mistake amongst 9mm owners – the wrong ammunition. There are 2 primary types of 9mm: 9×18 Makarov and 9×19 Luger. Although they are both 9mm, these rounds are not interchangeable.

There are many more firearms terms and calibers, but the above should cover the fundamentals. However, different parts of the world, and even within countries, use unique vocabularies to discuss their firearms. The most important lesson: If you don’t know what something means, ask. It is better to appear ignorant than to appear with a bullet in your foot.

The above chart shows some of the most common firearm calibers. For whichever firearm you choose, you must use the correct caliber of ammunition. If you try to load an anti-tank round into a pellet gun…it won’t work.

  • Invented in 1887, a .223 Remington is small, ideal for foxes, rabbits, possums, raccoons and other varmint. Note that .223 refers to a bore diameter that is .22 inches. This is the same size as a 5.56 mm. Because of its small size, a .22 creates very little recoil force in a firearm. The difference between a .22 and 5.56 is not initially obvious, even to experts, because the outer cartridges have the same diameter.
  • Invented in 1983, a 10mm round is primarily used in handguns. For handgun hunters, the 10mm can take down medium-sized game such as mountain lions, antelopes and deer. Of course, hunting with a handgun is very difficult.
  • Unveiled in 1925, the 270 Winchester is an extremely popular rifle cartridge. The ‘270’ refers to a bore diameter of .270 inches. Depending on the weight of the bullet used, a .270 can be used for varmint shooting or big-game hunting.

Rimfire vs Centerfire

A centerfire round contains a primer in the center of the cartridge. The cartridge ignites (fires) when the firing pin of the firearm strikes the center of the cartridge.

A rimfire round contains primer in the rim of the cartridge. It ignites when the firing pin strikes the rim of the cartridge.

Centerfire was invented circa 1810 and rimfire was invented circa 1845. Centerfire is more popular, and more powerful, and is generally used for military, police and self-defense purposes. Popular centerfire rounds include 9mm and 5.56/.223, as well as many handgun and rifle ammunition types and calibers.

Rimfire is less reliable than centerfire. Some popular calibers include .22 LR and .17HMR. Rimfire is less expensive than centerfire and is more popular with varmint hunting. Rimfire ammunition is also commonly used for training.

Rimfire (left) and Centerfire (right)

So, which is better? The answer depends entirely on your purpose. Centerfire rounds pack a much larger punch than rimfire. Rimfire is great for inexpensive small-game hunting, training, sport and limited self-defense. In competition, there are often separate divisions for centerfire and rimfire. Ultimately, the decision between centerfire and rimfire rests with the shooter. If you’re unsure, ask a professional for guidance.


Owning and operating firearms is a fundamental right in the United States. We do not receive, nor do we need, ‘permission’ from the government to use firearms. Firearms are spectacular tools that enable successful hunting, self-defense and resistance to government tyranny; nonetheless, merely having the ‘right’ to use firearms does not mean we can afford to be reckless.

Firearms can be extremely dangerous in the wrong hands and extremely helpful in the right hands. Understanding different calibers and chambering is intrinsic to understanding firearms. You should NEVER load ammunition into a firearm if you are not sure it will work. The correct caliber is often, but not always, etched onto the barrel of the firearm. If you are unsure, take the firearm to your local gunsmith. Firing the incorrect caliber can result in a severe malfunction which causes the firearm to never work again, and it can also cause severe injury or death.

For safe, productive firearms use, do your research and choose the correct caliber. Your family and friends will thank you for your safe, responsible exercise of the 2nd Amendment. In America, owning and operating firearms is a fundamental right – so please, exercise responsibility and conscientiousness.

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