How is a Caliber Measured? [4 Easiest Ways To Measure]

Are you confused about what should be the right caliber of a bullet for your gun? Are you thinking of buying shooting gear with a caliber that matches your hunting needs? Wanna shoot a crow and which rifle or handgun will do this job well?

Is your mind full of queries like that?

You are on the right page then. Here you will learn everything about the caliber of the firearm to accomplish your shooting dreams.

So go through how is a  Caliber measured? The caliber of any firearm can be measured by measuring the distance between high rifling points of the bore on the cartridge. It can also be measured by determining the distance between the opposite low points in the grooves. It actually depends on the design and dia of the bore of your firearm. Bore can be rifled or smooth both.

You can’t use the bullets of the same caliber in firearms with greater caliber. To understand this phenomenon it is essential to know about what is caliber actually and what is a bore?

Keep going!

What is  Caliber

Caliber stands for the diameter of both bullets and the gun’s internal bore or barrel. It is measured in inches and mm. Though caliber is not a SI measuring unit it is a way to measure the diameter of the internal bore of firearms.

It’s important to note here that you may read the caliber written on firearms with or without decimal. It can be .38 caliber or thirty-eight caliber.

The round off figure after decimal can be confusing. The reason is that this is not an exact measurement. There always have little difference from the exact number.

What is Bore

To understand Caliber, it’s important to know about the term bore as well. For firearms, a bore is an internal passage through which a bullet comes out. In rifles, it is grooved. However, handguns have twisted bores with groove cuts while shotguns have round bores without grooves.

The rifled grooves inside the bores let the bullets spin when they come out. Gun people use bullets that are fit to the bore of the gun.

Modern firearms have versatile bores while traditional bores were more smooth.

How To Measure The Caliber 

Measuring an unknown caliber of handgun, rifle or other ammo is an easy task. It will help you to estimate how deep your bullet will go inside the target to create a hole. The good news here is that firearms come with caliber marked on them. But if you buy traditional or used arms, you have to calculate it yourself because here caliber may be erased or not marked.

No worry, there are easy ways to do it. Let’s read on below:

Check Label On Gun: At first, read the label on the gun. Most of the modern firearms come with already published caliber. Modern rifles and handguns have an instructional menu for users. Here you can read the caliber. Check this factor before buying bullets also. You can read on the caliber box of bullets, grains, pellets, and shots. For example, in this Gamo Whisper, you can find the caliber marked on the body of a gun as shown in the pic.

How is a Caliber Measured? [4 Easiest Ways To Measure]

Use Vernier Caliper: You can use a vernier caliper to get the initial reading. However, to get an exact number in inches and centimeters you will have to do certain maths as shown in this video at the bottom of this content. In the end, you will get an approximate value, not the exact value.

How is a Caliber Measured? [4 Easiest Ways To Measure]

Measuring Distance between High Points: For rifles, the barrel is grooves and high points are visible on the cartridge. You can determine the caliber of the rifled bore by measuring the distance between the top rifling points. This is also known as the land measurement that is a more common way to determine caliber for rifles manufactured outside the USA.

Groove Measurement: For rifled barrels, firearms are grooved along the barrel. Modern ammo designed in the USA measures caliber by measuring the opposite low points in the grooves. This type of measurement is known as grooved measurement. However, groove measurement is always larger than land measurement.

There are many ways to put down the caliber as gun people write this number in millimeter and inch. Even some gun people write this number as hundreds of an inch, thousands of an inch or millimeter. For example, A bullet with 21 hundreds of an inch can be 0.21 in dia or 21 caliber bullet.

The caliber of each firearm is different depending on cartridge and bore. Firearms of the same caliber can have the same bullets but different cartridge.

For What Firearms We Use the Term Caliber

Understanding firearms is essential to buy the right gear. This will also help you to choose the bullet that fits well in your arm. Whenever you will hear the word firearm, there will be the next word Caliber.

I have already written everything about the caliber at the start of the article. But it’s important to note that not all firearms use this word Caliber for internal barrel measurement.

Gun people use the term Caliber for:

  • Rifles
  • Handguns
  • Ammunition

However, for shotguns, the internal bore of the barrel is measured in gauge.

Caliber Vs MM

Do Gun lovers often ask what is the difference between caliber and millimeter?

The fact is that Caliber is not a measuring unit like a millimeter. Caliber is just a way to tell what is the actual internal Barrel measurement of your gun.

 This measurement is often measured in inches. By doing a bit of maths, you can convert this measured caliber in inches into millimeters.

So, at first, let me make this clear that Caliber is not a standard measuring unit. However, mm is a standard measuring unit used to explain the internal diameter of the caliber.

By following the formula below, you can easily convert Caliber in inches into millimeters.

How To Convert Caliber into Millimeter

Usually, gun users like to measure  Caliber in inches. But what to do if they want to know it in millimeter. Though It’s easy you need to refresh your high school maths classes. Simply keep a few numbers in your mind and you can do it easily.

You should know

1-Inch =2.54cm

Now Convert 2.54cm Into Millimeter First

That is


We know that 




To Convert millimeters back into inches, divide the diameter to 25.4.

Get relief from this complicated Mathematics, use an online caliber to the millimeter converter. It will ease up your work!

Do Shotguns Have Caliber

No, hunting Shotguns are a bit different from rifles, handguns, and other ammunition. In Shotguns, the barrel is not rifled like handguns. It is typically not grooved finely to spin the bullet. That’ why it’s more round in shape as shown in the picture below:

How is a Caliber Measured? [4 Easiest Ways To Measure]

For this difference in barrel shape and size, we can’t use Caliber to measure the diameter of the barrel. Instead of Caliber, gunman use gauge to measure the exact diameter of barrel in shotguns. This is done by inserting lead balls inside the gun bore. Common shotgun gauges are 10-gauges,12-gauge,16-gauge, and 28 gauge. Remember, if the gauge is a smaller number, the shotgun would be of the larger bore diameter.

For start-ups, we recommend this shotgun as its best gear to practice hunting. You can read also our shotgun recoil pad guide.

How many Caliber is a 9mm

It’s already a known fact that caliber is measured in inches. Assume you are going to reverse mm in inches here. We know that

Caliber = 1-inch= 25.4mm

To determine the caliber in 9mm, you have to convert it into inches

9mm=0.354 rounded figure 0.38

So, there would be  0.38 Caliber in 9mm

However, this round of figures is different in different State rules.

This 9mm handgun is the best choice for shooters.

How big is a 50 Caliber Bullet

Gosh!50Cal bullets would be big enough.

The diameter of 50Cal would be around0.506: According to this big dia, the weight would be equal to 500 to 750 or more grains of bullets. It would be approximately 1 to 2 ounces. That is one big deal! It’s not the best choice for your rifle if you are planning to hunt small animals like squirrels, crows and birds.

If you had .50cal bullets or guns, it would be difficult to carry them. We recommend hunter’s pointe .50 Ammo Can to carry this big deal. It comes with a handle and contoured bottom.

Final Words 

Gun lovers, you should know well about caliber before purchasing any firearm. You can’t target little birds with large-caliber rifles or handguns.

 If you are into a variety of shooting gears, it’s obvious to consider the chart of the common bullet caliber size above in the article.

 But question yourself how a variety of bullets fit in one firearm. So you may need a firearm that fits well to your chosen caliber bullet or bullet that fits well to your shooting gear. It’s possible only if you are well aware of the caliber.