Understanding Firearms Caliber and Gauges | [Everything You Should Know]

Are you obsessed with a variety of firearms? If yes, you would hear different types of lingo when you go to buy arms or weapons.

If you are a fresher, you would get confused early. What to do when a dealer would question you how much should be the caliber of the gun you need to buy?

To weave the answer your mind would wonder! But if you go to the arms dealer after getting knowledge about the vocab related to ammunition, you can convince the seller easily.

For this, understanding firearms caliber and gauges is a must thing. There are so many types of firearms available nowadays like the rifle, shotgun, handguns, bullets, machine guns, pistol, etc. Caliber is a term used to measure the diameter of the bore of the rifled barrel. A gauge is used to determine the diameter of a shotgun with a smooth barrel.

Whenever you are buying firearms, you should consider your need, caliber of gun, and quality of the firearm as well. If you are going to get a firearm for hunting or small game shooting competition, you should buy handguns with low caliber.

Keep reading to know more!

What is Firearms

Before we move on to other topics, let’s first understand what is the proper definition of firearms.

Firearms are weapons with barrels that are specially designed to expel. It means that firearms are actually weapons made to expel bullets, shots, pellets, or projectiles. This expulsion may need an explosion in antique weapons.

Though it sounds destructive, firearms serve the purposes of security as well. The army uses these arms to fight against enemies to protect their territories. Hunters use these arms for small hunting games like legal birds and animal hunt.

Shooters also use these weapons to participate in big shooting games like target shooting competitions. So, if we use these firearms according to rules set by state they are not destructive. It’s up to use what makes them a good or bad thing.

Types of firearms

Nowadays there are several types of firearms available. Different types of these arms serve different purposes. For example, we cant shoot deer with low caliber shotguns. It depends on what actually we want to do from these arms.

Below is a brief detail of each type of firearm with images:

Rifle: It is a type of firearms with a relatively long barrel. The inner side of the barrel is rifled with grooves cut finely. This design ensures the spin of the bullet or projectile as it comes out.

Shotgun: It is a shoulder gun with a long barrel. Shotguns recoil pad are different from rifles in this respect that the inner side of the barrel is not grooved here. You can shoot a larger number of shots from this weapon.

Machine gun: This is a type of modern ammo. It is an automatic weapon in which the gun loads and fires until the trigger is pressed. It’s not for hunting purposes. It is mainly for wars.

Submachine gun: This gun is usually light in weight. It is a handheld gun that requires low energy to fire. It can be held on the shoulder.

Revolver: Revolver is a type of firearm. It has a chamber where bullets are installed. It is also a handheld small-sized weapon. Chambers are located in the cylinder. As soon as the trigger is pushed, the cylinder rotates and the bullet comes in the chamber to expel out with a bump.

Pistol: In pistols, instead of chambers you will see magazines for bullets. You need to pull the trigger to send the bullet from the magazine to the cartridge and then fire. There are so many variations in pistol designs.

Crafted Guns: The trend of manufacturing guns at home is still alive in this modern world. These guns are crafted by using old machines and techniques. These guns are pistols and shotguns.

Or even more advanced rifles. Antique guns also fall into this category.

Other types: There are other types of weapons also like air guns, gas guns, and lightweight weapons.

Understanding Parts of firearms

In order to Understand the terms Caliber and gauge, it is essential to know well about all parts of your firearms. For a new gunman, it’s a challenging task. To make it easy, its better to understand parts of each ammo you want to use. 

Below is a brief description of some basic parts of firearm:


These are moving parts of firearms. These parts are present in the gut of the gun. They help to load the gun, eject the bullet or shells.

These come in action when the gunman pushes the trigger. For example spring in the magazine, hammer, and cylinder in pistols.


The stock is an immovable part of the gun present at the end of each firearm. It is used to position your gun. For example, you place your gun on the shoulder with the help of stock. Another name of this part is the handle of a firearm.

It has two pieces

  • Butt
  • Fore-end

It is butt or buttstock that provides support to the weapon on your shoulder or hip. The fore-end is relatively not very thick like the butt and is located close to the buttstock. It also helps to position a firearm for target shooting


The barrel is a long tube through which the bullet is expelled. It is made up of durable metal. When the trigger is pushed, the gas force is created through a gun pin that pushes the bullet to travel through the barrel. This component is present in almost all firearms. In rifles of handguns, bullets pellets pass through the barrel. In shotguns, the barrel is smooth that allows only small shots to pass through.


This term is used for wide purposes. Even you have heard carpenters speaking about the bore. But here bore means the inner side of the firearm’s barrel through which the bullet comes out. Firearms can have a smooth-bore, twisted bore, or rifled- bore.

Traditional or antique shotguns come up with smooth-bores. This type of bore ensures a larger amount of bullets to come out easily.

The bore of rifles has grooves like cuts from inside. These grooves are drawn for the purpose to spin the projectile as it expels out. There are double bore rifles also.

By measuring the diameter of the bore, gunmen judge the right type of bullets for their firearms.


Gun people often confuse the term cartridge with a bullet. They use it instead of the bullet also. However, it’s not the right concept. The cartridge is a  full package that includes

  • Projectile
  • Ignition Device Or primer
  • The case made up of paper or plastic made to fit within the chamber

Depending on the type of primer, there are so many types of cartridge. A cartridge without a projectile is named as blank. The basic function of the cartridge is to provide energy to shot or bullet to travel further when it is fired.

What is the Caliber of Gun

When you go to a firearm dealer to buy any weapon, he would ask you about what should be the right caliber for your gun or bullet. Here you will be stuck if you dont know much about caliber. Read carefully if you’re planning to buy firearms in the future.

The caliber of gun, rifle, handgun, pistol, or airgun is a measurement of the internal diameter of the bore or barrel of the respective firearm.Whereas.The caliber of the bullet is a measurement of the diameter of the bullet, shot, pellet, or projectile. The standard unit of measuring the diameter of a bore or barrel is millimeter, hundreds of an inch, thousands of an inch depending on the metric rules of respective states(Europe or Us). Caliber is broadly expressed in inches. That’s why it’s not right to say I need a 9mm Caliber firearm. One more thing here caliber is not an exact figure it’s a round off number. So, you should always use the word approximate with caliber.

We come across .30-30 Cal Winchester .30-06Cal  Springfield, and a .308 Cal Winchester, all these can use .30cal projectile. If you find an extra number like .357 Magnum, it means .38Magnum. You can round off the figure to get the right caliber.

For modern firearms, the caliber is marked on the weapon. You need to measure caliber when your firearm is antique or caliber written on it is erased.

Note; For shotguns, caliber is not used.

How Caliber is Measured

It’s not as such a challenging task to know the unknown caliber of any firearm. For the latest firearms, caliber is always marked on the rear of the weapon.

You can measure the caliber by using the methods below:

Measuring the distance between high points: For rifles with groove cuts, you can measure the distance between two high points visible on the rifle or handgun bore. This type of measurement is called land measurement. It is a common way to determine the caliber for rifles manufactured in Europe or outside the USA.

Measuring the distance between the low point. For rifled barrels mostly manufactured in the USA, grooved measurement is used to determine caliber. In this method, the distance between two opposite low pints is measured in the groove. The value of groove measurement is always greater than the land measurement.

I would suggest using a vernier caliper to measure the diameter. You can also do a bit of maths to convert the diameter value from millimeter to inch. Here is an online conversion calculator for mm to -inch.

Read the post here for a detailed description of various methods to measure the caliber in detail.

understanding firearms caliber and gauges

Bullet Caliber Size Chart

The following is the chart of the commonly used caliber size chart. Here you can find the caliber metric values in inches and mm.

Common Bullet Caliber Size Chart:

Inch CaliberMetric CaliberCommon Bullet Diameter
.1724 mm0.172 inches
.20 , .2045 mm0.204 inches
.2215.45 mm0.221 inches
.225.6 mm0.223 inches
.2245.7 mm0.224 inches
.2436 mm0.243 inches
.256.35 mm0.257 inches
.266.5 mm0.264 inches
.276.8 mm0.277 inches
.2847 mm0.284 inches
.3087.62 mm 0.308 inches
.3117.9 mm0.311 inches
.3127.94 mm0.312 inches
.3238 mm0.323 inches
.338 8.6 mm0.338 inches
.3559 mm0.355 inches
.3569 mm0.356 inches
.3579 mm0.357 inches
.3639 mm0.363 inches
.3659.3 mm0.365 inches
.3759.5 mm0.375 inches
.4010 mm0.400 inches
.4410.9 mm0.429 inches
.4511.43 mm0.451 – 0.454 inches
.50 12.7 mm0.510 inches

Do we Use Caliber for All types of Firearms

An important query that gun lovers often ask!

The simple answer is that you can use caliber for rifles, pistols, handguns, crafted guns. machine guns.semi-automated  machine guns, rifles but not for shotguns.

Shotguns have a smooth barrel and we use a gauge to measure the dia of it. Caliber is mostly used for weapons with grooves or rifled barrels.

Here is a list of firearms that uses caliber:

  • Rifle
  • Pistol
  • Handgun
  • Machinegun
  • All weapons with rifled barrel

What is Gauge

Gun lovers often come across this term while purchasing shots for shotguns. This term is used for shotguns for small range shooting or hunting games. These guns have a smooth barrel to facilitate the flow of more and more shots rapidly.

Technically gauge is the number of lead balls of size approximately equal to the dia of the bore of a shotgun that it takes to weigh one pound lead.

These shotguns were manufactured many years ago. At that time, the gauge was used to measure the diameter of the barrel of shotguns instead of a caliber. 

Gauge is defined as the inner diameter of the bore of a shotgun. For decades, the gauge is still in use for shotguns only.

The smooth design of the shotgun’s barrel allows less spin of shots. For shooting startups, these are excellent weapons. 

A smaller gauge of shotgun means that the bore will be larger. That’s confusing! Read next to know how?

Gauge is used for

  • Shotguns 
  • Double bore Old rifles

How to measure Gauge of Shotgun

Gauge can’t be measured by vernier caliper or any other instrument. Gun people use lead balls of the same bore to measure it.

For gun people ease, the gauge is determined by calculating the number of lead balls of equal size of the bore of a shotgun that it takes to weigh 1 pound lead. For example, a 12 gauge shotgun does not mean the value of diameter in millimeters, inches, or centimeters. It means 12 lead balls of the same size equal to the bore, out of 1 pound of lead.

This method was discovered in the old days when the pound was the standard unit of measuring lead balls. In fact, gauge tells you about how many rounds you would make out of 1 pound of lead for your shotgun.

In general, the smaller the gauge, the larger would be the barrel. The largest shotgun comes in 4-gauge. Similarly, shotguns with 20-gauge will be smaller in the bore.

Guns with gauge .410 are exceptions to the rules. It has a diameter of 410/10000th of an inch which means 67 and a half gauge that is technically not the right gauge.

Shotguns will release shells or shots of the same gauge. For example, for a 12-gauge shotgun, the shells would also be of 12 gauge.

The good news is that like caliber, the gauge is also marked on the rear of the shotgun. You can also read it on the box.

Caliber Vs Gauge

Gauge is the diameter of the shotgun barrel only while Caliber is the diameter of the bore of the rifle, handguns, bullets, pistols, and other weapons with a rifled barrel.

Gauge is measured by using lead balls of the same size while caliber is measured by measuring the distance between low points or high points on the rifled barrel.

Gauge can’t be expressed in centimeters, millimeters, and inches as it is the number of lead balls of equal size of the bore of the gun, out of 1 pound of lead.

The gauge shows the exact number of lead balls while caliber gives an approximate round off figures.

Bigger caliber means the wider diameter of the barrel of the gun while the bigger the gauge number, the smaller will be the bore of the gun. For example, 20- gauge guns will be smaller in bore than 12 gauge shotgun.

However, both are similar in one respect that both measures the diameter of the bore or barrel of the gun.

Bottom Line

Well, we are about to wind up the article of understanding firearms. Hopefully, gun people learn something! 

It’s clear from the above description that without knowing much about Gauge and Caliber you can’t go too far in your shooting dreams. knowledge and research is the key to success whenever you are planning something new or different.

 The language of ammo is quite different and confusing for new learners. But with proper research, you can make yourself an expert shooter and firearm buyer.

How Caliber and Gauge confused you when you were a beginner? Tell us! We’d love to share your story here!

Bye for now, we’ll get back with the next article soon!