If you’re new to firearms or just curious about how bullets and shells are sized, you’ve probably come across terms like “caliber” and “gauge.” While they both describe the size of a firearm’s bore, they refer to very different systems. In this guide, we’ll explain what caliber and gauge mean, how they’re measured, and how each applies to different types of firearms.
What Is Caliber?
Caliber refers to the internal diameter of a firearm’s barrel, usually measured in inches or millimeters. It is most commonly used for handguns and rifles. When someone says a gun is “.30 caliber,” they mean the barrel has a diameter of approximately 0.30 inches.
Caliber can be expressed in two main formats:
- Inches: For example, .22, .38, .45 (common in the United States)
- Millimeters: For example, 9mm, 7.62mm, 5.56mm (common in military and European firearms)
So, a 9mm pistol and a .357 Magnum revolver are both classified by the diameter of the projectile they fire. The larger the number, the wider the bullet—though not necessarily the more powerful it is.
How Is Caliber Measured?
Caliber is typically measured from land to land in the rifling of a barrel, not from groove to groove. This gives the true diameter of the bullet the barrel is designed to fire.
For example:
- .22 caliber = 0.22 inches across
- .308 caliber = 0.308 inches across
- 9mm = 9 millimeters across (approximately 0.354 inches)
Note: Caliber alone doesn’t tell you the whole story. The type of cartridge, powder load, and barrel length all affect performance.
What Is Gauge?
Gauge is a measurement system used specifically for shotguns, and it works very differently from caliber. Instead of measuring bore diameter directly, it tells you how many lead balls of bore diameter would add up to one pound.
For example:
- 12 gauge: It takes 12 lead balls the size of the barrel to make one pound
- 20 gauge: It takes 20 smaller balls to make one pound
That means the smaller the gauge number, the larger the bore. So a 12-gauge shotgun has a larger diameter than a 20-gauge shotgun.
Gauge vs Caliber: Key Differences
| Feature | Caliber | Gauge |
|---|---|---|
| Used For | Rifles and handguns | Shotguns |
| Measurement | Barrel diameter in inches or mm | Number of same-size lead balls per pound |
| Units | Inches (e.g. .308) or mm (e.g. 9mm) | Whole numbers (e.g. 12 gauge) |
| Larger Number Means | Bigger barrel | Smaller barrel |
| Examples | .22 LR, .45 ACP, 9mm | 12 gauge, 20 gauge |
What About .410 Bore?
The .410 is an exception to the shotgun gauge system. It’s the only shotgun measured by caliber (in inches), not gauge. A .410 shotgun has a bore diameter of 0.410 inches, making it much smaller than a 12 or 20 gauge. It’s often used for small game or beginner shooters due to its lighter recoil.
Which Should You Choose: Caliber or Gauge?
Choosing between a rifle (caliber) and shotgun (gauge) depends on your intended use:
| Purpose | Recommended Type | Example |
|---|---|---|
| Target Shooting | Rifle (Caliber) | .22 LR, 9mm |
| Big Game Hunting | Rifle (Caliber) | .308, .30-06 |
| Bird Hunting | Shotgun (Gauge) | 12 gauge, 20 gauge |
| Home Defense | Either | 12 gauge shotgun or 9mm handgun |
Understanding Bullet vs Shell Size
Caliber refers to the bullet’s diameter, but cartridges also vary in length and load. For example, .223 Remington and 5.56 NATO are the same caliber but not always interchangeable due to chamber pressure differences.
Similarly, shotgun shells come in different lengths like 2 ¾”, 3″, and 3 ½”. You must ensure your shotgun is chambered to safely fire the shell length you’re using.
Frequently Asked Questions
Is a higher caliber more powerful?
Not always. While higher calibers fire larger bullets, power also depends on velocity, barrel length, and bullet design.
Which is better: 12 gauge or 20 gauge?
12 gauge is more powerful and versatile, but 20 gauge offers less recoil and is better for beginners or smaller shooters.
Can I use 20 gauge shells in a 12 gauge shotgun?
No. You should only use shells that match your shotgun’s chamber size and gauge. Mixing gauges is dangerous.
Why are rifles measured in caliber and shotguns in gauge?
It’s due to tradition and function. Rifles and handguns are designed for precision, so exact diameter (caliber) matters. Shotguns fire pellets or slugs, where bore volume is more relevant than diameter.
Conclusion: Know Your Bore to Shoot Better
Understanding caliber and gauge is essential for choosing the right firearm and ammunition. Caliber applies to rifles and handguns and is measured in inches or millimeters. Gauge applies to shotguns and is based on the weight of lead balls that fit the bore. Knowing the difference helps you stay safe, shoot more accurately, and pick the right tool for your purpose.
Ready to explore more about firearms and ballistics? Check out our complete guides to ammo types, zeroing scopes, and firearm safety to become a smarter shooter.