Understanding Bullet Calibers | OnPoint
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understanding bullet calibers

Understanding Bullet Calibers: What You Need to Know

Caliber can refer to either the internal diameter of the barrel of a firearm or the diameter of the bullet. Bullet refers to the metal projectile when the gun fires, while cartridge refers to the entire unit (including the bullet, propellant, and primer) in the firearm. It is important to ensure the caliber of your firearm and ammunition match to avoid damaging your firearm. 

Naming Conventions

Although naming conventions for firearms and their corresponding cartridges have changed over time, in general, modern firearm calibers refer to the name/dimension of the cartridge the firearm is chambered for, and the firearm’s barrel bore diameter. In the United States, bullet calibers are often in millimeters or by hundredths of an inch. For example, in the United States a .45 caliber firearm has a barrel diameter of 0.45 inches and is compatible with a .45 caliber bullet. Likewise, a 9mm pistol has a barrel diameter of 9 millimeters and is compatible with a 9mm bullet. 

Most Popular Calibers 

.22 Rimfire

Commonly known as a “twenty-two,” .22 Rimfire is a popular caliber among shooting enthusiasts and is the most commonly sold ammo in the world. Given the small size of the bullet and the reduction in firepower, .22 caliber firearms generally have less recoil than larger caliber firearms. These firearms come in handy for small game hunting and target shooting. 

.380 ACP

.380 ACP caliber firearms are popular due to their low recoil and small size. The .380 ACP caliber or 9MM Kurtz (short) bullet is a lower power and shorter version of the 9MM round. 0.380 ACP round is popular with people looking for lower recoil or highly concealable firearms and is thought to have less stopping power that other self-defense rounds. It is best for targets at relatively short ranges. 


9mm, 9x19MM Parabellum or 9MM Luger bullets are identical and are different in name only. The 9MM round is the most common handgun round in the world today exceeding all other self-defense combined and as a result it is also the most cost-effective round and comes in many variations for both target shooting and self-defense. 9MM is the NATO standard handgun round and in service with many militaries and police forces worldwide. 9MM dominance in the word today leads not to the question of why use 9MM but rather why not.

Most Popular Calibers, Continued

.38 Special

The .38 Special is a revolver round and was the standard caliber used in service firearms by US police forces from the 1920s through the 1990s. The .38 Special and .357 Magnum are almost identical in size–although both calibers have equivalent diameters, the length of the cartridges differ.2 Due to their smaller length, .38 Special bullets can be safely used in .357 Magnum chambered firearms; however the converse is not true.5 .357 Magnum caliber bullets cannot be safely used in a .38 Special chambered firearm because the .357 is much more powerful.

.357 Magnum

 .357 Magnum caliber bullets are the same diameter as .380 ACP and .38 Special bullets, however, the cartridges differ in length. Originally designed in the 1930s, it was a step up from the .38 Special and used by police departments for penetrating steel car doors and ballistic vests, a feature the .38 Special was unable to offer.  Although the .357 Magnum has a stronger recoil than .38 Special, it became popular due to its superior stopping power. As such, the .357 Magnum is useful for self-defense and sometimes for small and medium game hunting.6

Most Popular Calibers, Continued

.223 Remington and 5.56×45 mm

.223 Remington and 5.56x45mm (5.56 NATO) rifle bullet calibers are similar in size and sometimes used interchangeably in 5.56 NATO caliber rifles. Since 5.56 NATO bullets have more pressure than .223 bullets, either .223 or 5.56 NATO bullets can be used in a 5.56 NATO caliber rifle. However, 5.56 NATO bullets cannot be used in a .223 rifle, as the greater pressure may damage the firearm or cause harm to the shooter. Recommendations for appropriate ammunition caliber should be checked against the firearm’s owner’s manual or the barrel. Both .223 and 5.56 NATO are popular calibers for AR-15 style rifles. 

.308 Winchester and 7.62x51mm

.308 Winchester (.308) is the civilian equivalent of the 7.62x51mm (7.62 NATO) round in use by the US military. 0.308 is a popular caliber for hunting rifles for its significant firepower and extended range. The .308 may be one of the best all-around hunting rounds today and, in the hands of a competent shooter, appropriate for all but the largest animals in North America.

 .40 S&W

 .40 S&W was previously for use by the FBI and was subsequently popular with law enforcement in North America. Today, the FBI and an increasing number of law enforcement are moving to the 9MM because it is more cost effective, has higher round capacities in firearms, less recoil and comes in varieties that are ballistically equivalent. .40 S&W remains a great defense round though its popularity is rapidly shrinking.

Most Popular Calibers, Continued

 10 MM

Originally used in the late 1980s by the FBI for its superior stopping power, it was later abandoned by the agency for the .40 S&W due to its larger size and recoil.8 However, 10 mm firearms remain ideal for experienced shooters for hunting or protection against big game in the backcountry, particularly grizzly bears. The 10 mm caliber is available in a relatively small number of handguns including the Glock 20, SIG Sauer P220 Legion and some models of the well-known 1911 platform.7 Due to its large recoil, 10 mm chambered firearms are generally most suitable for experienced shooters.

.45 ACP (Automatic Colt Pistol)

The .45 ACP caliber bullets were designed for the 1911 pistol for the US Military. Though in use in many modern firearms and such venerable classics as the Thompson sub-machine it will almost always be interchangeable with the 1911 pistol. John Moses Browning designed both for each other. Both were in service with the US Military from 1911 to the mid-1980’s when they moved to 9MM. The .45 has superior stopping power and moderate recoil.  

How do I Know Which Caliber Bullet my Firearm Takes?

Boxes contain caliber measurement along with each owner’s manual for the corresponding firearm. The owner’s manual and barrel of your firearm both state which bullet calibers are appropriate for your chosen firearm. Failure to use the correct bullet caliber can result in damage to your firearm and bodily injury to yourself and others.


1 Wikipedia contributors. (2023, February 6). Cartridge (firearms). Wikipedia. https://en.wikipedia.org/wiki/Cartridge_(firearms)

2 Amped-Marketing. (2021, May 5). Bullet Caliber Guide | Bullet Caliber Size Chart by The Hub. The Hub Tucson. https://thehubaz.com/blog/the-basics-of-calibers/

3 Basic Bullet EXPLAINED: Sizes, Calibers, and Types – {MUST READ}. (2017, October 16). TheGunZone. https://thegunzone.com/bullet-sizes-calibers-and-types/

4 Wikipedia contributors. (2023b, February 24). .38 Special. Wikipedia. https://en.wikipedia.org/wiki/.38_Special

5 .357 Magnum vs .38 Special. (n.d.). Diffen. https://www.diffen.com/difference/.357_Magnum_vs_.38_Special

6 Wikipedia contributors. (2023c, February 26). .357 Magnum. Wikipedia. https://en.wikipedia.org/wiki/.357_Magnum

7 O., & Mudgett, C. (2021, August 18). Why the 10mm Is the Ultimate Handgun Cartridge for Hunting and Personal Defense (Plus 8 of the Best Pistols You Can Buy). Outdoor Life. https://www.outdoorlife.com/story/guns/the-10mm-is-the-ultimate-handgun-cartridge-for-hunting-and-personal-defense/

8 McHale, T. (2019b, July 16). 10mm Ammunition: 7 Things You Need to Know. AmmoLand Shooting Sports News. https://www.ammoland.com/2019/07/10mm-ammunition-7-things-you-need-to-know/

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