I thought I'd share this info with you all (I hope it wasn't posted elsewhere already). Some of you might already know, but I didn't, so I figure at least someone else out there doesn't know.
Growing up, I always knew that the smaller the shotgun gauge, the bigger the shotgun shell was. That's all I knew, and I took it for granted.
If you've ever wondered why a 12-gauge shotgun is 12-gauge, and why a 28-gauge is 28-gauge, here's why:
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The Gauge, bore or calibre of a shotgun is a unit of measurement used to express the diameter of the barrel. The gauge or bore of the inside diameter of a barrel corresponds with the number of identical solid spheres that can be made from a pound of lead.
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In other words, a 12-gauge shotgun has a 0.729-inch diameter bore, because it takes 12 0.792-inch diameter lead spheres to make 1 pound of lead.
The same goes for a 20-gauge. It takes 20 0.615-inch diameter lead spheres to make 1 pound of lead, and thus a 20-guage shotgun has the bore diameter of 1 of those spheres.
This goes for every shotgun gauge (10,12,16,20,24,28,32), except .410. The .410 shotgun is actually a measurement of caliber, meaning the ball leaving the barrel is 0.41" in diameter. If the .410 shotgun were to be measured in gauge, it would be a 67.5-gauge shotgun.
Also, old cannons were also sometimes measured in gauge, although their measurements were based off of the weight of iron, rather than lead.
Just some facts, I hope someone learned something
Source:
Gauge (bore diameter) - Wikipedia, the free encyclopedia 
