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Thomas C. Tabor is Gun World’s resident hunting columnist, and routinely goes afield in pursuit of small birds, large game and good stories.
Reader Question: I don’t understand how the gauge of a shotgun is determined. With rifles and pistols the caliber designation is frequently based on the diameter of the bore with the larger the number, the larger the caliber. But with shotguns, the reverse seems to be the case with the larger the number of the gauge, the smaller the bore diameter. What does the gauge designation mean and how is it determined?
—Ralph F., Bend, Oregon
Tom Tabor Answers: You are right…metallic cartridges are commonly named based on the diameter of the bullet they shoot. For example, a 6mm Remington rifle cartridge contains a bullet measuring 6mm in diameter and a .30-06 contains a bullet reflective of the first number in its name (.30 inches, or more precisely, .308 inches). Of course, not all cartridges possess a hyphenated name as in the .30-06, but in this case ’06’ relates to the year that that cartridge was developed (1906) and the .30 to the diameter of the bore.
Determining the gauge of a shotgun is not nearly so straight forward, but still is based somewhat loosely on the bore diameter. But rather than simply taking a measurement across the bore, an archaic method dating back to long before the introduction of smokeless powders, or even before self-contained ammunition entered the scene, is used. In this case, the gauge determination is based on how many round lead balls it would take of the same diameter to make up a pound in weight. In other words, it would take 20 of those lead balls matching the diameter of a 20 gauge shotgun bore to make up a weight of 1-pound and it would take 12 lead balls the diameter of a 12-gauge bore to equal one pound.
There is one major exception to this form of classification, and that is the .410 shotgun. In this case, the name actually does reflect the bore diameter in inches, that being .410”. Because of this, anyone referring to a shotgun as being a 410 bore would be in error. Properly speaking, a 410 should only be referred to as that, “a 410” or a “410 shotgun.” I know this all seems unnecessarily complicated and confusing, but it is no less perplexing than how muzzle velocities of shotshells are sometimes presented. Again, holding onto a primitive method of classification, some ammo manufacturers continue to stick to the age-old rating of dram equivalent (Dr. Eq.). This again dates back to when the muzzleloading shotguns were loaded with black powder and the powder charges were measured in drams. When smokeless powder entered the scene, ammo manufacturers must have thought that shooters would relate better to the shell’s performance if they kept the designation in the same basic vernacular as the black powder loads. So, the term dram equivalent was born, meaning for example, that a modern day smokeless powder shotshell rated at 3 Dr. Eq. would produce a muzzle velocity equal to that of a shotshell previously loaded with 3 drams of black powder. Ever so slowly, shotshell manufacturers are beginning to move away from the Dr. Eq. rating system in favor of the actual muzzle velocity. Nevertheless, even in today’s 21st century this age old rating system persists to some degree.