Definition of Shotgun "Gauge" or "Bore" in Firearms Terminology

Shotgun with smoke coming out on black background
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Most shotguns are categorized by a gauge measurement, although in some countries, the same measurement is known as the gun's bore.  A shotgun's gauge (bore) can seem confusing at first, since the smaller the gun's gauge, the bigger the gun. This can seem counterintuitive at first, but here's an explanation of how it is calculated: 

Gauge is determined by calculating how many round lead balls equal to the diameter of the gun's internal bore it takes to equal 1 pound in weight.

So the smaller the gun's barrel bore, the more lead balls it would take to equal a pound. A 20-gauge shotgun, for example, has a smaller-diameter bore than a 12-gauge, and hence it takes 20 round balls equal to the gun's bore diameter to make up 1 pound, while the larger gun requires only 12 round balls. 

Another way to look at it: if the spherical round lead ball that fits the diameter of the gun is 1/12 of a pound, the shotgun is a 12-gauge. Similarly,  a 10-gauge round ball weighs 1/10 lb, while a 28-gauge round ball weighs 1/28 lb.

Here is a chart showing the relationship between gauge and diameter of one ball fitting the gun's bore for the most common shotgun gauges:

10-gauge             .78" (19.7mm)

12-gauge             .73" (18.5mm)

16-gauge            .66" (16.8mm)

20-gauge            .62" (15.6mm)

28-gauge            .55 (14.0mm)

There are some unique other gauges around, but they are mostly obsolete and sometimes are considered collector items.

They range from large 4-gauge and 8-gauge guns to light 32-gauge guns.

Most often, gauges are specified in even numbers, but this seems to be mostly a matter of convenience. When gunsmiths were coming up with these standards long ago, they preferred dividing by an even number rather than an odd one.

The Exception

A few shotguns are measured by diameter rather than gauge. The most common of these is the .410 shotgun, the smallest shotgun normally used in hunting. Technically it is appropriate to know this type of gun as ".410 bore," not a ".410 gauge." If a .410 shotgun were to be calculated as other shotguns are, it would be roughly a 67-gauge.