Why do Shotguns use Gauge Instead of Caliber?
I’ve been working on a bunch of guides for the Year of the Shotgun. As I was working, there was a question that kept coming up in my brain. Namely, why are rifles and pistols measured in “caliber” while shotguns use “gauge”? I mean, if you really want to get pedantic about it, gauge technically is the caliber of the shotgun. You don’t really hear people refer to shotguns as a “12 gauge caliber” gun. Maybe sometimes a “large caliber” shotgun, perhaps.
Clearly, some research was in order…
For starters, both “gauge” and “caliber” deal with the size of the barrel of the gun. The big difference between the two concerns how they are calculated. Concerning “caliber” in the non-pedantic sense, it is simply the diameter of the bullet what passes through it. So a .45 caliber bullet is so called because it is .45 inches in diameter or thereabouts. A 12 gauge shell, however, isn’t .12 inches across because that would be smaller than a .22LR round.
The thing is, a shotgun shell is a completely different beast. You’re not moving a single solid projectile through the barrel, you’re moving a whole bunch of tiny solid projectiles and many felt a different measurement was needed. Even though the slug-style of shotgun ammo wasn’t used at the time, they turned to cannons for their inspiration. Did you know that the “gauge” measurement itself originates from the old black powder cannons? They used to refer to a cannon’s size by the weight of the iron cannonball shot through it. Iron was a pretty consistent weight and so saying a “six pound” cannon meant it would shoot 6 pound cannonballs from it. Later on “gauge”, as a standard, came to mean the weight of a solid sphere of lead that could pass through the barrel (measured in fractions of a pound). That means that a 12 gauge shotgun could allow a 1/12 pound ball of lead through it. It’s actually very similar to the way they figure out the gauge of hypodermic needles.
The powers that be have since refined the measurement even more to be, according to the great Wikipedia, “a ball of lead (density 11.352 g/cm3 or 6.562 oz/in3) with that diameter has a mass equal to 1/n part of the mass of the international avoirdupois pound (453.59237 grams).”
If you want to get really accurate, there’s a formula to calculate it (n = the bore of the gun):
Go ahead and try it out on your own!
So ultimately, because of the “wad of stuff” style of ammo that shotguns used, it would be near impossible to go off of the diameter alone because that diameter doesn’t give the whole picture. Granted, it still doesn’t but because the gauge measurement is more about volume than actual diameter, it’s a better measurement for shotgun ammo. It’s still just a starting point. Very rarely do you see shotgun ammo expressed only as the gauge beyond just a “high level” description. You might see “3 inch 12 gauge double-aught buck” or “2 ¼ inch 10 gauge bird” on boxes. Because of the near exponential number of combinations available, gauge alone isn’t enough to describe the ammo you need for your gun. As to what those extra bits of info mean, well, that’s another guide.
Images licensed under Public Domain via Wikipedia.
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