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
Eyrie Productions, Unlimited http://www.eyrie-productions.com/
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