A 20 gauge is a smaller bore than a 12 gauge, even though 20 is the bigger number. That surprises most people. Gauge counts how many lead balls fit in a pound, so a higher number means a smaller ball and a smaller bore. The two are not interchangeable, and putting a 20 gauge shell into a 12 gauge gun is one of the best known mix-ups in shooting.
This post explains what gauge means, compares real 12 and 20 gauge loads from one maker, and covers the safety rule that applies to every shotgun. It is general information, not advice for a specific gun.
What “Gauge” Actually Means
Gauge is an old measure. Federal Premium describes it this way: a gun maker would take lead balls the same diameter as the shotgun bore and count how many of those balls it takes to equal one pound. A 12 gauge bore takes 12 balls to make a pound.
Follow the same logic for a 20 gauge. It takes 20 balls to make a pound, so each ball has to be smaller. That is why the bigger number is the smaller bore. It is also why the numbers look backwards at first glance.
The one exception is the .410. It is measured in inches, the way rifle and pistol calibers are, and not by the pound rule.
Federal says it loads shotshells in 10, 12, 16, 20 and 28 gauge and .410, and that shells range from 1 3/4 to 3 1/2 inches long. A shell’s gauge and length are both printed on it, and both matter. The source for all of this is Federal’s own guide, Shotshell Ammunition Basics.
12 Gauge vs 20 Gauge in Real Factory Loads
Numbers on a page are easier to read than a description. Here are two target loads from Federal’s Top Gun line. Both use the same shell length and the same shot size, so the only real difference is the gauge.
| Load | Shell length | Shot size | Shot weight | Velocity |
|---|---|---|---|---|
| Federal Top Gun 12 gauge | 2 3/4 inch | 8 | 1 1/8 ounce | 1,200 feet per second |
| Federal Top Gun 20 gauge | 2 3/4 inch | 8 | 7/8 ounce | 1,210 feet per second |
Sources: Federal’s Top Gun 12 gauge and Top Gun 20 gauge pages, read November 2026.
The speeds are nearly the same. The 20 gauge carries about 22 percent less shot, 7/8 ounce against 1 1/8 ounce. That fits what you would expect from a smaller bore. Shot size 8 is a small pellet, and the same size is used in both shells.
Do not read too much into one pair of loads. Both gauges are made in many weights and speeds, and some loads in each gauge are far heavier than these. The point here is how the gauge relates to the shell, not a ranking of which one is better.
How to Choose Between Them
There is no single right gauge. People choose based on what they plan to shoot, who will be using it, and how much weight and kick they want to handle. A few honest things to think about before you walk in:
- Who will shoot it? A person who wants a lighter load may look at 20 gauge. The Federal numbers above show less shot in the 20 gauge shell, but how a gun feels depends on the gun, its weight and the load, so try the shotgun in your hands.
- What loads are available for it? Ask what the counter can show you for each gauge and each shell length.
- What is the gun’s chamber length? Look at the barrel. Many shotguns are marked with a chamber length, such as 2 3/4 inch or 3 inch. The next section explains why this matters.
- Do you already own ammunition? If you have a box of one gauge, that decides which gun it goes with.
If you are comparing guns in person, hold each one, and bring a friend who has shot them if you can. Our post on buckshot vs birdshot is a good next read once you know the gauge, because the pellet type changes what the shell does.
The Rule That Matters Most: Never Mix Gauges
SAAMI, the Sporting Arms and Ammunition Manufacturers’ Institute, is the group that writes the standards for ammunition and chambers. Its interchangeability guide is blunt: different gauges are never interchangeable.
It explains the worst case. A 20 gauge shell can fit into a 12 gauge shotgun and lodge in the forcing cone of the 12 gauge barrel. If a 12 gauge shell is then fired behind it, that shell meets what SAAMI calls “essentially a plugged bore.” The mistake has its own name in the industry, the “12/20 burst.” SAAMI notes that other mixes can do the same thing, such as a 16 gauge shell in a 12 gauge chamber, a 20 gauge shell in a 16 gauge chamber, or a 28 gauge shell in a 20 gauge chamber.
The National Shooting Sports Foundation (NSSF) puts it in one line: it only takes one cartridge of improper caliber or gauge to wreck your gun. That is from its article Only Use the Correct Ammunition for Your Firearm.
The practical habits are simple:
- Read the gauge printed on the barrel before you buy or load anything.
- Read the gauge and length on the box and on the shell.
- Keep only one gauge at your bench or in your bag when you can.
- Examine every shell you put in the gun.
Shell Length: The Second Rule
Gauge is not the only match you have to make. SAAMI also covers length. A shorter shell in a longer chamber of the same gauge is acceptable, such as a 12 gauge 2 3/4 inch shell in a 12 gauge 3 inch or 3 1/2 inch chamber. The reverse is not true. A longer shell in a shorter chamber can create higher than expected pressure, because the mouth of the shell opens into the forcing cone and makes a constriction.
SAAMI still tells owners to check with the firearm maker if the gun is not marked for the shorter shell. When you are not sure, read the owner’s manual and the markings on the barrel. The full guide is here: SAAMI Generally Accepted Firearms and Ammunition Interchangeability.
SAAMI’s storage and handling pamphlet adds a short rule that fits here. Make sure the designation on the ammunition matches the designation marked on the firearm’s barrel, with this example: a 12 gauge shotshell is to be used in a shotgun marked 12 gauge.
Next Step at Lakeside
Not sure of the gauge on a shotgun you own or inherited? Bring the gun by, or tell us the make and model, and we will read the markings with you. If you are shopping for one, say how you plan to use it and we will talk through your options at the counter. You can also send us a message first. For more about the store, start from our home page.
