Measuring a Shotgun Bore
Shotgun bore size is described by gauge, a traditional system that works very differently from the caliber measurements used for rifled bores. Instead of stating a diameter directly, gauge encodes bore size through the weight of a lead sphere. Learning the logic behind gauge clears up why shotgun numbers seem to run backward compared with rifle calibers.
How Gauge Is Defined
Gauge is defined by how many identical lead spheres, each exactly the diameter of the bore, together weigh one pound. If it takes twelve such spheres to make a pound, the bore is 12 gauge; if it takes twenty, it is 20 gauge. Because a smaller sphere means more of them fit in a pound, a larger gauge number corresponds to a smaller bore, an inverse relationship that surprises newcomers.
Gauge Versus Caliber
Caliber states a bore or bullet diameter directly, in inches or millimeters, so a larger caliber number means a larger bore. Gauge is the opposite: it is a count derived from sphere weight, so the number shrinks as the bore grows. The two systems answer the same question, bore size, but along inverted scales, which is why they should never be read the same way.
The Bore Diameter Behind Each Gauge
Each gauge corresponds to a specific bore diameter fixed by the sphere definition. A 12 gauge bore measures roughly 0.729 inches, while a 20 gauge measures about 0.615 inches. These diameters are held to standard, so a 12 gauge barrel from any maker accepts a 12 gauge shell, and the gauge number is a reliable stand-in for a precise dimension.
The .410 Exception
The .410 is the well-known break in the pattern because it is named by its bore diameter in inches, about four hundred ten thousandths, rather than by a sphere count. Expressed as a true gauge it would be roughly 67 gauge, but that number is never used. The .410 is a bore-caliber designation living among gauge names, and it is the exception students are expected to remember.
Worked Example
Compare a 12 gauge with a 20 gauge. Twelve bore-diameter lead spheres weigh a pound in the 12 gauge, giving a bore near 0.729 inches; it takes twenty spheres in the 20 gauge, giving a smaller bore near 0.615 inches. So the 12 gauge, with the smaller number, is the larger, more powerful bore, and the 20 gauge, with the larger number, is the smaller bore. The counting definition is what flips the intuition.
A Common Misconception
The classic error is assuming a bigger gauge number means a bigger shotgun, as it would with caliber. It is the reverse. A 10 gauge is larger than a 12, which is larger than a 20, because the number counts spheres per pound rather than measuring diameter. Only the .410 is named by its actual bore size, which is exactly why it stands out as the exception.