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A 12-gauge shotgun, nominally 18.5 mm (0.73 in), can range from a tight 18 mm (0.71 in) to an extreme overbore of 20 mm (0.79 in). Some also claim an increased velocity with the overbored barrels, up to 15 m/s (49 ft/s), which is due to the larger swept volume of the overbored barrel.
The 20-gauge shotgun, also known as 20 bore, is a type of smoothbore shotgun. 20-gauge shotguns have a bore diameter of .615 in (15.6 mm), while the 12-gauge has a bore diameter of .729 in (18.5 mm). [ 2 ] 12-gauge and 20-gauge shotguns are the most popular gauges in the United States .
The gauge number is determined by the weight, in fractions of a pound, of a solid sphere of lead with a diameter equal to the inside diameter of the barrel. So, a 10-gauge shotgun nominally should have an inside diameter equal to that of a sphere made from one-tenth of a pound of lead. Each gauge has a set caliber.
Thus, a 10-gauge shotgun has a larger-diameter barrel than a 12-gauge shotgun, which has a larger-diameter barrel than a 20-gauge shotgun, and so forth. M35 .410 shotgun cartridges for M-6 survival gun w/.22 long rifle for comparison. The most popular shotgun gauge by far is 12-gauge.
In the case of a 12-gauge (18.5 mm) shotgun, it would take 12 spheres the size of the shotgun's bore to equal a pound. [12] A numerically larger gauge indicates a smaller barrel: a 20-gauge (15.6 mm) shotgun requires more spheres to equal a pound; therefore, its barrel is smaller than the 12-gauge. This metric is used in Russia as "caliber ...
For example, the barrel of a 12-gauge shotgun is equal to the diameter of a 1/12 of a pound lead ball (0.729 in) and a 20-gauge can fit a 1/20 pound lead ball (0.615 in). Using this method, a .410 bore is equivalent to a (hypothetical) 67-gauge, instead of the incorrectly labeled 36 gauge (0.506) in Europe and South America. [14] [15]
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The exit end of a choke is smaller by some dimension than the actual bore of the barrel. This difference in diameter is the amount of constriction. For example, for a 12 gauge, the bore diameter of the barrel is nominally 18.5 mm (0.73 in), although different manufacturers do vary their as-manufactured bore from this diameter slightly.