Understanding Bullet Sizes, Types and Calibers

If you’ve just recently begun searching for a gun, it’s easy to become overwhelmed by the plethora of bullet sizes, types, and calibers on the market. 

Look no further than this complete guide to understanding bullet sizes, the types of bullets you can pick from, and the primary caliber sizes available.

The different types of calibers and bullet sizes

The first thing you need to know is that a firearm’s caliber is simply the diameter of the gun’s barrel, and therefore the diameter or bullet sizes that will pass through. The bullet is the metal projectile component or merely the metal tip, while the compartment in its entirety is a cartridge

The following are the 11 primary caliber sizes on the market:

  • .380
  • .22LR
  • .40 S&W
  • 9 mm
  • 10 mm
  • .45ACP
  • .38Spl
  • 5.7x28mm
  • .30 Carbine
  • .357 Mag
  • .300 Blackout
  • 7.62x39mm
  • 5.56x 45mm
  • .223
  • 7.62x54mmR
  • .30-06
  • .50 BMG
  • 12 ga

The different reference points noted above, such as “mm” for “millimeter” or “ACP” for “Automatic Cartridge Pistol” are all units of measurement. Different caliber sizes are measured with different units, so some could be in inches and others in millimeters. There is another weight unit known as a grain which indicates the bullet weight. Grains are incredibly small, with 7,000 grains totaling only a single pound. 

Common caliber types

Let’s take a look at some of the most popular bullet sizes with gun aficionados everywhere.

.380 ACP

A .380 ACP is one of the larger bullet sizes available, sometimes known as a 9mm Short. It’s become widely popular in recent years to load pocket pistols with. The .380 bullet provides fairly insignificant recoil and penetrates well at a close range. 


The .22 rifle features one of the most widely used calibers, with a bullet between 30 and 40 grains. The .22 bullet makes for pretty moderate shooting in rifles and pistols alike, with almost no recoil, which makes it a great choice for new shooters. 

The .22LR bullets are also very cost-effective, which is yet another reason they are the favored choice for beginners. They are a few notches above a round of pellets and are primarily used for killing birds, snakes, and rats. 

.40 S&W

The .40 was originally manufactured for the FBI as a somewhat lesser 10mm cartridge and has been used extensively by law enforcement agencies for years. The .40 definitely has more kick to it, with bullets ranging in weight from 155 to 165 or 180 grams.


Classic 9mms have the same bullet sizes as the bullets in the .380 and .38 Special. The 9mm bullet types are standard issue for NATO countries and law enforcement officials around the globe. These bullets weigh anywhere from 115 to 147 grains. 

.45 ACP

The .45 ACP is a mammoth bullet with impressive stopping power (the ability to incapacitate a target) and has been used by law enforcement and the military for many years. The bullet is about 230 grains weight-wise with mild recoil. However, we wouldn’t recommend that you select a .45 ACP if you are brand new to wielding firearms.

.38 Special

Shooters typically use the .38 Special in revolvers. The bullet has a noticeable recoil that could be a bit of a challenge for newbies using a light revolver. The .38 has a lengthier cartridge and higher powder quantities, but the bullet moves slower and weighs more than the 9mm does.

A .357 Magnum is almost the same except that it is a little longer. You can fire a .38 Special bullet in a .357 Magnum firearm, but not vice versa. These bullet types weigh anywhere from 110 to 132 or 158 grams.

.308 and 7.62x51mm

These two bullet sizes are almost identical. Pros sometimes mix rounds, but unless you are a seasoned shooter, you should use the round designed for your particular rifle. The .308 is widely popular for hunting with a mild recoil and sports significant stopping power. Bullets range from 150 to 208 grains.

.223 and 5.56x45mm

These two bullet types are also nearly the same, but the 5.56 has more pressure than the 2.23. What this means is that you can fire the .223 bullets in a 5.56 firearm, but not vice versa. The bullets are roughly 55 grains, and you can expect a much lighter recoil with the cartridge.

The primary types of bullets

Before you head out to the shooting range or on a hunting expedition, these are the primary types of bullets and terminology you’ll need to know.

Full Metal Jacket (FMJ)

The most common kind of bullet is the full metal jacket or FMJ, featuring a soft metal core like lead encased in a tougher metal like copper. FMJs are usually round or pointy, but some can be flat. An FMJ usually creates a minute wound channel and penetrates the target, making it a good option for range practice. 

Pro tip: Remember to pack your gun and bullets in a gun range bag when traveling to and from the range.

Open Tip (OTM)

Open-tip bullets or OTMs are the ammo of choice for long-distance. They offer consistent rounds and accuracy from many hundreds of yards away.

Hollow Point (HP)

Hollow points are designed to enlarge once they hit their target or another object. Many shooters with concealed weapon permits and law enforcement officers rely on the HP as their round of choice.

Soft Point

Soft point bullets were designed to try and achieve the ballistic qualities of the FMJ but with more augmentation. With soft point bullets, some of the lead is visible at the bullet’s tip, which then flattens when it reaches the target. 

Ballistic Tip

A ballistic tip bullet is a hybrid of the stopping power the hollow point provides and the movement of an FMJ. Ballistic tips have a hollow point design encased in plastic and are ideal for hunting. 

Bird Shot

 Good for hunting clay pigeons and birds, bird shot are bullets with dozens of small pellets encased in each shell.


Slugs are single projectile bullets featuring roughly an ounce of tough metal with high stopping power. While they don’t expand as bird shot does, a seasoned shooter could achieve accuracy as far out as 100 yards. 

FAQs About Bullet Sizes, Types and Calibers

What are the most common bullet sizes and calibers used in firearms?

The most common bullet sizes and calibers used in firearms are .22, 9mm, .38 Special, .357 Magnum, .40 S&W, 10mm Auto, .45 ACP/Auto, 5.56 NATO/223 Remington and 7.62x39mm/.308 Winchester.

How do different bullet weights affect accuracy and performance?

Bullet weights affect accuracy and performance by affecting the speed at which they travel down range as well as their trajectory or “drop” over distance; lighter bullets tend to travel faster but with more drop while heavier bullets tend to be slower but have less drop over distance.

Are some types of ammunition more dangerous than others due to the size or shape of the bullets they use?

Some types of ammunition may be more dangerous than others due to the shape of the bullet itself (hollow points) or its construction (armor piercing); these types of ammunition can cause excessive tissue damage when shot through flesh or armor plating respectively which makes them better suited for self-defense applications rather than recreational shooting activities such as target practice or hunting wildlife where less lethal options should be used instead if possible. 5. Caliber refers to the internal diameter measurement of a cartridge case measured either in millimeters (ex: 9mm) or inches (ex: 0.45).

Do different types of firearms require specific types or sizes of ammunition for safe operation?

Yes, different types of firearms require specific types or sizes of ammunition for safe operation since some guns may not be designed to handle certain kinds of projectiles due to barrel length restrictions amongst other things like magazine capacity etc. Different cartridges also have varying amounts powder charge associated with them depending on what type it is; always make sure you’re using an appropriate round for your gun before firing!

Can a gun be loaded with multiple caliber rounds at once, or must each round have its own separate chamber for firing?

A gun cannot usually be loaded with multiple caliber rounds at once because each round must have its own separate chamber for firing; however certain revolvers feature interchangeable cylinders that allow shooters switch out one cylinder containing multiple calibers quickly without having take apart the whole gun – this would still depend on whether all respective chambers were made out same size though so check your firearm owner’s manual first!

Wrapping up

It takes time and patience to learn and understand the different bullet sizes, types, and calibers available, but hopefully this guide has given you a solid introduction to help you decide which is right for your shooting habits. If you’re just starting out, .22LR bullets are a worthy contender to consider. They are easy on the budget, have very little recoil, and make for great hunting rounds on a slightly smaller scale with quality velocity and accuracy.


About the Author

Trey is a lifelong hunter and avid camper. He lives outside Denver, CO with his wife Kaci and their lab mix Ziggy. They spend as much time as possible outdoors - hunting, fishing, and camping.

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