Many years ago, on one of the campus newsgroups at WPI, a few of us got into a colossal flamewar with some dude who claimed to be a firearms expert and who, based on his posts, knew basically nothing. One of his most insistent claims, of which he steadfastly and with increasing vehemence declined to be disabused, was that he possessed (illegally, on campus) a ".12-gauge shotgun", and that those of us who told him the decimal point was misplaced there were idiots.

(This is the guy, BTW, who originated the GweepCo joke "reply if you want, I usually respond with my fists." Nowadays he's probably one of those creeps who contrive to send the SWAT team to the homes of people who disagree with him on Reddit.)

Anyway, here's why what he was saying was stupid. It's true that the calibers of many cartridges, particularly those developed in the United States and Britain, are designated in decimal inches. A .45-caliber bullet, for instance, will have a nominal diameter somewhere in the neighborhood of 45 hundredths of an inch. (They're rarely exactly as specified.) The smallest such caliber you are ever likely to encounter in a firearm is .177, which is an absurdly small projectile.

Now let's consider, for a moment, the bore of a 12-gauge shotgun. It is nearly three-quarters of an inch in diameter. Not twelve-hundredths of one. This is because, with the exception of the teeny .410 (not "410 gauge"), shotguns are not traditionally measured by caliber; they're measured in gauge, and gauge is one of those weird systems of measurement that only the English could have thought was a sensible way to do things.

Basically, what the gauge of a firearm's bore means is—get ready for this—the number of spheres you can make out of a pound of lead such that each will have a diameter equal to that of the bore. No, seriously, that's what it means. If you have 12 lead spheres of the diameter of a 12-gauge shotgun's bore, they add up to a pound. A 20-gauge bore's diameter would make 20 balls out of the same pound of lead. And so on. This is why a 20-gauge shotgun is smaller than a 12-gauge.

Shotguns—and some big-game rifles—used to be available in a wide range of different bores or gauges. Back in the days of the intrepid explorers, a few British gunmakers made 2-bore rifles, which were basically small shoulder-fired artillery pieces, and which tended to be used on things like elephants and rhinos. Nowadays, one tends to find shotguns only in 12- or 20-gauge; very occasionally 16- or 10-gauge, though I'm not sure anyone makes those new any more. .410 is also still around, filling a niche as a light-duty shotshell for dealing with things like snakes.

Making things even more complicated in the shotgun ammunition nomenclature game is the fact that most shotgun shells are loaded with, well, shot, and shot has its own arcane sizing system. Actually it has about a dozen different, mutually incompatible arcane sizing systems. In the US this is pretty much arbitrary—by definition, a US No. 2 shot is 0.150" in diameter, while the same size shot in Britain is designated "B" and defined as the size of which you get 87 out of a pound of lead. (There's that pound of lead again.) Meanwhile, a European No. 2 is .148", which is size 6 in Sweden. And so on. And that's just bird shot! Buckshot is vaguely similar, but the numbers are all different and the British version is actually based on the old naval standards for sizing grapeshot. The shot gets larger as the number gets smaller, until they get to 0, and then the bigger ones get more zeroes. It's all fairly baffling.

(There is such a thing as a shotgun shell that has a single full-width projectile in it, probably-not-surprisingly called a shotgun slug. Since shotguns aren't rifled, these usually have rifling on them, which is not as effective as having it built into the gun, but better than just launching it out of a smoothbore tube. There are also—bizarrely enough—rifled shotguns, out of which one can no longer fire shot, but which will take discarding-sabot shells containing bullets smaller than a 12-gauge bore. There is apparently a velocity gain to be had here, I don't really understand the advantage over just using a rifle. As far as I know, no one makes fin-stabilized discarding-sabot 12-gauge sub-diameter slugs, though that would be hilarious. And expensive!)

Anyway, yeah. .12-gauge indeed. (An actual .12 gauge bore is theoretically possible, but that would really be a slightly oversized 8-pounder field gun. I'm not going to bother to do spherical volume calculations based on the density of lead and figure out how large the bore would be.)

Shotgun shells are kind of interesting, just because of the way they've evolved. In some ways they're actually more like a self-contained muzzleloader charge than most modern cartridges, because they still have an inert wad between the powder charge and the payload (and often one in front of the payload as well). Also, because they (usually) don't have a single full-diameter projectile, the hull needs to be the full length of the cartridge—or really longer than that, so there's extra material to close it off at the front and keep the shot from just rolling out.

They used to be all brass, like most other cartridges, but that gets pretty expensive owing to the amount of metal involved. Then, for a long time, they were mostly paper with a brass head and usually a brass "cuff" extending up from the head, to reinforce the section that contains the actual powder charge. I'm just old enough to remember there were still some old paper shells kicking around when I was a kid; nowadays they're virtually always plastic with a brass head (and still the brass reinforcement for the powder section). They were also once color-coded by gauge, which is why many 12-gauge shells on the market nowadays tend to be red and 20-gauge ones yellow, though this is no longer de rigueur and as far as I know was never technically required.

Also, note that they're no longer universally manufactured with lead shot, because it occurred to someone eventually that hundreds of people spraying small particles of lead around the countryside (particularly wetlands) every fall for decades on end was, in retrospect, probably not the best idea. Nowadays it's usually steel or—weirdly—bismuth.

None of this information is really critical to understand what a shotgun is or does, but as I've had folks express an interest in the nuts and bolts of ammunition types and whatnot before, I figured it would be a good time to jot down a few notes, since we haven't really looked at one before.

--G.
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Benjamin D. Hutchins, Co-Founder, Editor-in-Chief, & Forum Mod
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	-={(Astynax)}=- "I just explained the shotgun gauge determination to my wife, who proceeded to look at me as if I had just sprouted a second head, and said head had offended her -greatly-."

Bore, or gauge, comes from rifles, really, and not shotguns, primarily.
Back in the day of the early British Empire, gentlemen would go hunting in the back of the bundu. They would DO so with single, or more commonly, double barrel rifles firing black powder. And the thing about such rifles is, there is really little way to increase the power. You have a notable limit on the velocity of the round. So, the only real way to increase the power of the round is to increase the WEIGHT. And so, you increase the barrel to go along with it. By putting barrel reaching up towards inches in diameter, you can put a BULLET that wide in the gun.
Why would you want to, though? Well, 8-6-4 bore(4 being really the last 'reasonable' size) is there to be a stopping rifle. You don't, per se, go hunting with that...but when your standard hunting rifle isn't up to the task, and you have a SERIOUSLY pissed off rhino, buffalo, bull elephant bearing down on you at a VERY high clip with blood, potentially literally, in it's eye? You really, *really* want something that will do the job right fucking NOW!!!

Of course, smokeless powders came along, and the limit of how fast you could drive a bullet became a thing of the past, and so the high-bore(low-bore?) guns disappeared. But the system of measurement remained in shotguns, which had also been made in bore-size, because a shotgun functions differently, and so there was no reason to change that idea.

Today, 12-gauge is indeed the standard, 20-gauge is almost entirely a bird-hunting size, and some companies still make 10- and 16-gauges, Browning among them.

The .410 is quite popular among some for defensive purposes. This is because the base of the .410 is the same size as the .45 Colt(note, NOT the .45 ACP), and so, if you extend the cylinder, you can load both rounds into a revolver. The revolver is still classified as a handgun, though, and not a shotgun, and so, you can get around the 'minimum length of barrel/gun' issue. Their actual effectiveness is very debatable, though.

And finally, the formula for bore-size. The density of lead is 6.55 ounces per cubic inch. That in turn gets us 0.41 pounds per cubic inch. If we invert that, we get cubic inches per pound, instead, so 1/0.41, which gets us 2.4.
So, 2.4 cubic inches per pound, convenient for a 12-gauge, that is 0.2 cubic inches for each sphere. Now, using the basic formula for the volume of a sphere, 4/3*pi*r^3, we get a radius of 0.363 inches, which gives a bore measure of 0.73(rounded).

Other sizes bore makes it a bit more difficult, but not all that much.
.410 counts out to about 67.6 gauge, which of course is the reason it is read as .410. And a .12 gauge would be...3.4 inches, approximately. THERE is a gun for you! Somewhere between A½ and AA punt gun, and WELL above the 4-bore stopping guns above, and, of course, NOT something you could even carry, much less shoot.

Oh, I forgot...yeah. There is a mad French company that DO make FSDS 12-gauge rounds. The idea/advantage is, of course, as ALL sabot-rounds, that you get a round that is powered by a full load of powder, while weighing half as much as it 'should', producing one hellacious amount of velocity(for those that aren't aware of why sabots are a thing).
And the reason for a rifled shotgun is really(lack of) range, meant for areas where the greater range of a rifle is potentially dangerous, and so you have "shotgun-only" hunting zones. Or so I'm given to understand. Around here, at least, there is no such thing.

...!
Gott's Leetle Feesh in Trousers!

A little bit of fiddling with Wolfram Alpha says 8.6 cm, or about 3.4 inches.

A light 6-lbr, (in the old system) actually, because they were measured based on iron shot. The modern system would have it as a 23-lbr or so, being based on AP shot rather than a cannonball.

There's your large gauge shotguns.

The Royal Armory in Leeds has a collection of these. I'd heard of them, but seeing them is something different. That's a big shot gun. Documentation claimed some were deployed as anti-U-boat vessels during WWI and WWII, but says none saw action.

I can see a passel of these clearing a U-boat's deck gun in a hurry.

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-Pasha
"Don't change the subject"
"Too slow, already did."

There was a Facebook game where one of my go-to tools for a while was the "Bolt-action 4-bore." I never understood what made them choose that name, but now I have something like an idea.

Peter Eng
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How many bears could Bear Grylls grill, if Bear Grylls could grill bears?