Even if you haven’t got any previous experience with shotguns, chances are you’re already familiar with the term “gauge.” 

After all, shotguns are often referred to in relation to their gauge in movies, TV shows, and the news. “Gauge” refers to the barrel’s bore size – in other words, the barrel’s inside diameter. 

It’s a common mistake to think this means a shotgun’s gauge is exactly the same thing as a rifle or handgun’s caliber, as “caliber” refers to the bore size and the diameter of a rifle, handgun, and even machine gun’s ammunition. 

However, the gauge number doesn’t actually refer to the size of the bore itself! 

Instead, it denotes the weight (in factions of a pound) of a solid sphere of lead with a diameter equal to that of the bore. 

So, for example, a 12 gauge shotgun doesn’t have a 12-inch diameter (imagine that!). 

Instead, a 12 gauge shotgun’s barrel has the same diameter as a lead ball that weighs 1/12th of a pound. The actual diameter is 0.725-inches.

Confusing? You’re not alone!

Shotgun gauges can be very confusing when you first learn about them, especially because it’s our natural tendency to think of things in relation to something similar.

In the case of gauges, the caliber of other firearms is what we naturally want to compare them to. As mentioned earlier, however, this isn’t an accurate comparison to make. 

Gauge simply isn’t relatable to how rifles and handguns are measured by caliber, even though both practices rely on the barrel’s inner measurements.

Adding to the confusion is the fact the smaller the gauge number, the larger the shotguns bore. Again, our natural tendency is to think big number equal bigger bores. 

Just remember gauge refers to weight as a fraction of 1 pound.

Another way to think of it is to imagine having a handful of lead balls with the same diameter as the shotgun’s bore. The gauge refers to how many of those balls you need to make a pound. 

So a 20 gauge shotgun requires 20 lead balls, while a 12 gauge shotgun only needs 12. The smaller the bore’s diameter, the more balls you’ll need. 

Does this make more sense?

So why do we measure shotgun gauges like this instead of using the same system as rifles and firearms? 

To understand this, we need to take a quick look at history.

From Hand Cannons to Cannons to Muzzle-Loaded Shotguns

In the early 900s, toward the end of the Tang Dynasty (618 – 907 CE), the Chinese started using gunpowder as a military asset

By the Yuan Dynasty (1279 – 1368 CE), they were using small bombards that used gunpowder to propel a projectile (typically small stones and pieces of pottery and metal). 

They were also being made large enough to be considered cannons, which had to be placed on a wheeled cart. 

The smaller bombards are sometime cited as the first handguns, but they’re more accurately referred to as miniature or hand cannons and compared to shotguns.

Because of this close history between cannons and shotguns, we adopted the gauge measurement from the way cannons are classified. 

Cannons were (and still are) classified according to their projectile’s mass. 

Thus a 12 pound cannon, or 12 pounder, shot cannonballs that weighed approximately 12 pounds. The weight of the cannonball obviously depends on its diameter – in other words, the wider the cannon’s bore, the heavier the cannonball it fired.

But that’s not all! 

When shotguns were first developed as a weapon distinct from the hand cannon, they were still muzzle-loaded and used black powder. 

They were basically cannons fired from the shoulder, so it made sense to continue using the same classification system. 

However, these shotguns fired projectiles that weighed less than 1 pound, so they had to come up with a different name – imagine having to describe your shotgun as a 12th pounder because its projectiles weighed 1/12th of a pound!

This is how we came up with gauge as a way to classify shotguns according to their bore!

Common Shotgun Gauges

Luckily, you don’t need to worry about remembering (or even thinking about) as many shotgun gauges as you would if you were memorizing rifle and handgun calibers. 

There are far fewer gauges to consider and only 2 or 3 of them are common enough to warrant focusing on when starting out with a shotgun for home defense.

Here are the 6 most popular shotgun gauges you’re likely to come across:

  • .410 – notice something funny about this gauge? That’s right; it looks similar to how calibers are described. The .410 gauge is the only one where this happens, and it’s not by coincidence. 

This gauge is so small it’s easier to classify it by diameter than mass, the same way rifle and handgun calibers are. 

Being the smallest option for shotguns, the .410 is generally favored by younger shooters because it has a more forgiving nature and low recoil (the spring back from the force of being fired). 

This makes it unsuitable for home defense use though, even for young shooters. It’s better used as a small sporting shotgun where your targets are fowl and/or small game.

  • 28 Gauge – this is probably the least used gauge, as it’s also rather small. For this reason, you’re better off leaving the 28 gauge alone unless you’re looking for a slightly stronger sporting shotgun than the .410.
  • 20 Gauge – the next largest gauge after the 28, a 20 gauge shotgun will pack a decent punch while still being forgiving to the shooter thanks to its relatively light recoil. 

Generally speaking, it’s a good entry-level shotgun gauge for young or diffident users looking for a home defense weapon, especially as there are a variety of shell options available. 

However, shotgun experts such as Bob Brister still recommend using the 20 gauge as a sporting firearm rather than as a home defense weapon.

  • 16 Gauge – if you’re looking for a better mid-point between the lighter feel and recoil of the smaller gauges above and the power of the more popular 12 gauge, then the 16 gauge might be the shotgun for you. 

The 16 gauge was more popular in the first half of the 20th century, mainly as a bird gun. Even though it does offer a decent mid-point, it’s not ideal as a home defense weapon for 2 very important reasons. 

First, its drop in popularity means there’s a relatively low selection of shells available to choose from. Second, and more important, most 16 gauge shotguns are single shot designs. 

For home defense, you’ll need something capable of firing a few successive shots before needing to reload.

  • 12 Gauge – the 12 gauge is arguably the most popular (and certainly best known) shotgun, so it should come as no surprise to hear it’s also the shotgun of choice for home defense needs. 

Its popularity as a favorite for the military, law enforcement, and citizens alike means the majority of shell sales are for 12 gauge shotguns. It also has the widest variety of gun styles, shell varieties, and accessories available.

  • 10 Gauge – finally, we have the 10 gauge. Although 10 gauge shotguns have a bigger barrel than those listed above, it isn’t necessarily a more powerful weapon. 

It certainly isn’t a common gauge to see manufactured anymore either, mainly due to its bulkiness and overall weight. 

While the 10 gauge shotgun does still have a purpose, they aren’t an appropriate weapon for home defense for those same reasons.

Nominal vs. Actual Bore Size by Gauge

While it isn’t an important consideration, you might find yourself interested in how the different gauges translate into bore diameter. 

Rather than sit trying to figure out what kind of mathematical equations you’ll need to use to do so, you can just reference this handy list:

  • .410 bore = 0.410 inches
  • 28 gauge = 0.545 inches
  • 20 gauge = 0.615 inches
  • 16 gauge = 0.665 inches
  • 12 gauge = 0.725 inches
  • 10 gauge = 0.775 inches

These are based on the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) standards. 

It’s also important to note these are for a smooth barrel bore and have a deviation tolerance of +0.020 inches. 

So the actual diameter of your shotgun’s bore might be slightly different, especially if you’re comparing different shotgun brands to each other!

Length of the Shell

Because it’s the most popular (and best-recommended) for home defense, all the information in this book from here to the conclusion will be given in reference to the 12 gauge shotgun.

It’s important to remember gauge only describes the bore diameter of your shotgun. Shells have many other characteristics you should keep in mind, including their length.

The length of the shell is usually given in inches and describes the length of the fired case, not the unfired case. 

It’s very important for you to be familiar with this distinction, as you want to be sure your home defense shotgun can accept all available shell lengths.

Shotguns will generally have their gauge number and chamber length stamped onto the barrel. This chamber length tells you the maximum shell length that can be safely fitted into your shotgun’s chamber. 

Remember: the shell length given on the box is how long the shell’s casing will be after being fired. Before being fired, it will actually be a little shorter, sometimes by up to 0.5-inches.

It is extremely important to never use a longer shell than what is marked on the barrel, but a shorter shell is generally safe to use. 

Firing or attempting to fire a shell longer than your shotgun’s chamber can and will generate pressure in your firearm that’s dangerously higher than it was designed to handle. 

This creates a significant safety risk and will damage your shotgun, potentially injuring yourself and others around you.

With that very important warning out of the way, here are the most common lengths for 12 gauge shells:

  • 2 ¾” – the tried, tested, and true option; has been around the longest and offers the most choice for combination of loads for buckshot and slugs 
  • 3” – the ‘magnum load’ before the 3 ½” came out; longer more powerful load than the 2 ¾”, with a good selection of loads 
  • 3 ½” – the largest of the 12 gauge shells; powerful, contains a lot of shot, and offers lots of load options

Bear in mind bigger is not always better. 

The strong kick-back from a 3 ½” shotgun shell makes it harder to take a properly placed second shot right away, which can often be crucial in a home defense situation.

Our best recommendation for a home defense shotgun is a 3” chambered 12 gauge shotgun, using mostly 2 ¾” shells for better recoil control.