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Shotgun Gauges and Shot Sizes

By Hunters Guide. Checked against SAAMI technical publications on September 18, 2026.

Gauge counts how many lead balls of bore diameter it takes to weigh one pound, which is why a bigger gauge number means a smaller bore. Shot size numbering runs backwards too, for a similar reason. Both catch people out.

What shotgun gauge actually measures

Gauge is a way of stating the diameter of a shotgun bore as a count rather than as a measurement. SAAMI defines it as "A term used in the identification of most shotgun bores. (410 bore is an exception.) It is related to the number of bore diameter lead balls weighing one pound".

It stops being strange once you work it through. Picture a lead ball cast to exactly fill a shotgun's bore. Now count how many of those balls it takes to make one pound.

Twelve? That barrel is a 12 gauge. If it takes twenty, each ball has to be lighter, which means smaller, which means the bore it came from is narrower. That's a 20 gauge.

That's the whole trick: the number is a divisor. Bigger number, smaller share of a pound per ball, smaller hole.

Why are 20-gauge shells yellow?

Yellow is reserved industry-wide for 20-gauge shotshell bodies, and no other gauge uses it. It exists because of one specific mix-up, the kind that destroys shotguns and hands. Walk through it step by step. The sequence is the point.

1. The 20 drops into a 12-gauge chamber. It's narrower and slides straight in with no resistance, so there's nothing to notice.
2. It stops at the forcing cone. That's where the chamber steps down to bore diameter, and the 20's rim catches on the step. Now the shell sits ahead of the chamber, out of sight, held there by a shoulder of steel.
3. You close the action on what feels like an empty gun. Perhaps it clicks and nothing happens. Or you never registered the first shell going in at all.
4. You feed a 12-gauge shell. This one seats properly. The action locks, and it fires exactly as designed.

Except the barrel is plugged. Several thousand pounds of pressure per square inch slam into a lead-and-plastic obstruction that can't get out of the way fast enough, and the barrel lets go right there. Directly under your forward hand.

SAAMI doesn't soften it: "Different gauges are NEVER interchangeable, and some mix-ups are known to have the potential to cause catastrophic results". It happens often enough to have earned its own name: the 12/20 burst.

It isn't theoretical, either. Two people share a shell bag in a blind. A shotgun gets passed around at hunt camp. One shell from last season has been riding loose in a coat pocket, or a father and son are shooting different gauges side by side.

All ordinary. And 12 and 20 isn't the only bad pairing: SAAMI lists a 16 in a 12-gauge chamber, a 20 in a 16, and a 28 in a 20 as doing the same thing, just less often.

The yellow hull is a real safety device, then, not branding. Keep one gauge per pocket and one per bag. Glance at the colour before anything goes into a chamber. And if a shotgun ever goes click with no bang, unload it and open the action, then look down the bore from the breech end before you feed it anything else.

Why the .410 is measured in inches

The .410 is named for its bore diameter in inches rather than by a ball count. It is the one common shotgun size that breaks the pattern, and SAAMI flags it as the exception right in the definition. Mind the wording too. It's .410 bore, not 410 gauge. Say the second one out loud in a gun shop and you'll get a look.

Convert it anyway, if you're curious. A lead ball 0.410 inches across weighs roughly one sixty-seventh of a pound, which makes the .410 about a 67 gauge. Nobody calls it that. By the time bores got that small, the inch measurement had already stuck.

Why shot size numbering runs backwards

Shot size is a number that describes the diameter of the individual pellets. It runs backwards because the number gets subtracted, not added. There's a real formula behind it. Most people never get told that part.

SAAMI gives the arithmetic as "Shot Size = (17 - Number Designation)/100". In other words, start at seventeen and subtract the shot number. Read what's left as hundredths of an inch. For number 4 shot, seventeen minus four is thirteen, so 0.13 inches. Number 8 leaves nine, or 0.09 inches.

So the number doesn't name the pellet. It names how far down from seventeen the pellet sits. Number 8 is always smaller than number 4, and once the subtraction is in your head you'll never need to memorise the order again.

Two caveats. Half sizes exist (number 7 and a half is the common one) and sit between the whole numbers. Buckshot and the lettered sizes above number 2, the BB and B and T range, run on separate scales. The subtraction doesn't apply to them.

The trade between pattern density and pellet energy

Pattern density is how many pellets arrive inside the spread at a given distance. Pellet energy is what each single pellet is carrying when it gets there. Since a shell holds a roughly fixed weight of shot, the two compete directly, and picking a shot size means picking where you sit between them.

Split an ounce of lead into 410 tiny pellets and the pattern is thick and forgiving, with very little behind each hit. Ninety large pellets from the same ounce? Each one carries real energy. But gaps open in the pattern, and they widen with every metre. No size gives you both, which is why the whole range exists.

Small, fast targets up close call for higher numbers. For larger, tougher quarry at longer range, go lower: fewer pellets, each one heavier.

Your choke shifts things too, and so does the particular load. Pattern your own shotgun on paper at the distance you actually shoot. It'll tell you more than any table.

Non-toxic shot is a separate matter. Some situations call for it instead of lead, and those requirements live in the regulations, not in a study guide, so check the rules where you hunt. Want the reps? Our ammunition practice questions drill the component and sizing concepts. For what shells cost per round by gauge, see our ammo price comparison.

Questions about gauges and shot sizes

Does a bigger gauge number mean a bigger shotgun?

No, it’s the opposite. The number counts how many bore-diameter lead balls make a pound. A bigger number means each ball is lighter and smaller, so the bore is narrower. A 10 gauge is wider than a 12, while a 28 gauge is much narrower than a 20.

Is .410 the same thing as 410 gauge?

No. The .410 takes its name from its bore diameter in inches, so the proper term is .410 bore, not 410 gauge. Put it through the gauge formula and it lands somewhere near 67 gauge. Nobody uses that number.

Which is smaller, number 4 shot or number 8 shot?

Number 8. Take 17, subtract the shot number, and you have the pellet diameter in hundredths of an inch. Number 4 works out to 0.13 inches. Number 8 is 0.09 inches. Higher number, smaller pellet.

Can you fire a 20-gauge shell in a 12-gauge shotgun?

Never. A 20-gauge shell drops into the larger chamber and hangs up at the forcing cone instead of falling out the muzzle, so the chamber looks empty. A 12-gauge shell loaded behind it then fires into a plugged barrel, and the barrel bursts.

Why are 20-gauge shells the only yellow ones?

The industry reserved that colour for 20 gauge alone, specifically so a 20 can’t hide in a handful of shells of another gauge. No other shotshell body is yellow. In a pocket or a shell bag, the odd one out shows at a glance.