Shotguns are generally measured by gauge rather than caliber, differing from handguns and rifles. The gauge is determined by how many lead balls sized to the exact diameter of the bore it takes to make one pound. For example, one pound takes 16 lead balls the same diameter as a 16-gauge shotgun bore or 28 lead balls the same diameter as a 28-gauge shotgun.

The lower the gauge number, the larger the bore diameter. Common shotgun gauges are 10 gauge, 12 gauge, 16 gauge, 20 gauge and 28 gauge.

There is only one shotgun classified by caliber: the four-ten. The four-ten is designated as .410 and is a true 41 caliber. This designation also is present for rifles and handguns.

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