When buying a shotgun the very first thing you find out is that the measurement of the “caliber” of a shotgun is not the same as the measurement of the caliber of a rifle or pistol.

 As a matter of fact there are no shotgun “calibers”; they are measured by what is called a Gauge.

 Checking out the American Dictionary I found that a “Gauge” where it applies to shotguns is a traditional unit measuring the interior diameter of a shotgun barrel. The gauge of a shotgun was historically defined as the number of lead balls, each of a size just fitting inside the barrel, that were required to make up a pound.

In other words, if a lead ball fit perfectly into the barrel of a shotgun and twelve of those balls were required to make up one pound that weapon is a 12-gauge shotgun.  Using this principle you can expect a 20 gauge to require 20 balls to make a pound, a 16 gauge would require 16 lead balls and so on.

 That’s great but who has the time or the desire to take a bunch of lead balls and ram them down a shotgun to determine what gauge it is?

 To make it easier I have found this table which lists the internal diameters for each gauge number of shotgun.

Reference; World Almanac & Book of Facts 1996

The one and only exception to this rule is for any shotgun bore diameter under .5 inches (1/2 inch); these are defined by the actual inch measurement of the shotgun bore diameter .  

For example a shotgun with its bore measured at .41 inches is a .41 shotgun, we call it a “four ten”, and list it as a .410.

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