Shotgun gauges are determined by the number of lead balls of a given diameter required to make one pound of that size ball. Thus 10 balls of 10 gauge diameter are required to make one pound of such balls, or 20 balls of 20 gauge diameter are required to make one pound, and so forth. This is the traditional, and very old, system. The actual (nominal) bore diameters of the various gauges are as follows: 10 gauge = .775 inch, 12 gauge = .729 inch, 16 gauge = .662 inch, 20 gauge = .615 inch, 28 gauge = .550 inch. The .410 is named for its nominal bore size, and is not a gauge at all.

The last couple decades of the 19th Century were a time of great change in the shotgun world, in terms of both guns and shells. Guns went from twist barrels to fluid steel, choke boring had appeared in the late 1870's, and external hammer double guns became hammerless. Self contained shot shells went from brass hulls to paper hulls. In 1880 the 10 gauge rivaled the 12 gauge as an all-around gauge, and the 8 gauge was fairly popular for waterfowl hunting. The 16 gauge was the specialized upland gauge, and the 20 was regarded as something of a curiosity. 10 gauge shells were loaded with 1 1/4 ounces of shot, and 12 gauge shells with 1 ounce.

As time passed and the 19th Century became the 20th Century, both guns and shot shells became more efficient, and the smaller gauges made great inroads on the popularity of the large gauges. By the 1890's the 12 gauge had begun to replace the 10 gauge as the all-around gauge, and by 1900 the 12 gauge was firmly in first place.

By the 1920's, new progressive burning powders allowed heavier shot payloads, and all of the shotgun gauges took a step up in effectiveness. The 12 gauge was first in sales and the smaller 16 and 20 gauges were gaining in popularity and had passed the 10 gauge. 12 gauge shells were encroaching on the 10 gauge shell's payload, just as the 16 and 20 gauges were encroaching on the 12 gauge's traditional payload. The use of the 8 gauge, even for waterfowl hunting, had declined to the point that when it was banned for such use by the Federal Firearms Act of 1938 practically no one cared. Eight gauge guns were simply too heavy and clumsy to be practical.

By the 1950's, according to Jack O'Connor in his The Shotgun Book, the 12 gauge had about 50% of the market. The 16 gauge had about 25%, with the 20 gauge gaining in popularity and up to 17%. The little .410 bore accounted for about 4% of sales, and the 10 gauge and 28 gauge each had about 2% of the market.

In the 1960's plastic shells began replacing the traditional paper hulls, and first shot protectors, then one piece wads, were incorporated into shotgun shells. These advances provided better patterns across the board by reducing the amount of shot deformed on its trip down the barrel. The smaller gauges, especially the 20 gauge, particularly benefited from the improved efficiency and continued to climb in popularity.

By the 1970's, again according to Jack O'Connor, the sales of 12 gauge guns and shells had declined somewhat to less than 50% of the market. The 20 gauge had taken over second place with about 20% of the market, and was still increasing its market share. The 20 had become the favored upland gauge, and the 20 gauge 3" magnum load with 1 1/4 ounce of shot (the same as the traditional 12 gauge "duck and pheasant" load) had become popular for waterfowl. The 16 was in third place, with something less than 20% of the market, and fading. The .410 was up to 10% and the 28 and 10 gauges brought up the rear, with the 10 still slipping.

The 12 gauge was the top all-around and waterfowl gauge, and its 3" Magnum load with 1 7/8 ounces of shot almost gave it the performance of the nearly obsolete 10 gauge, which was up to a full 2 ounces of shot in a 3 1/2" magnum shell. The 2 3/4" 16 gauge magnum shell handled 1 1/8 ounce of shot, as did the 2 3/4" 20 gauge magnum shell. A full 1 ounce of shot was being stuffed into little 2 3/4" 28 gauge shell, and the 3" .410 shell carried 3/4 ounce of shot.

Then the US government stepped in with their toxic shot legislation. Lead, of course, is not toxic unless somehow introduced into the system, but facts are not important to the anti-gun do-gooders in government, and there was some evidence (from seriously flawed studies) that waterfowl were picking up expended shot from the bottom of shallow ponds and suffering lead poisoning. Thus the government mandated inefficient steel (or other non-lead) shot for waterfowl hunting, and pretty much put the blocks to the 20 as an all-around (upland and waterfowl) gauge. Even the big 3" magnum 12 gauge load could carry only 1 1/8 ounces of the new shot, and became a marginal killer on high flying ducks and geese. This gave the 10 gauge, whose fat 3 1/2" magnum shell could carry 1 3/8 ounces of steel shot, a new lease on life.

Subsequently, to bolster the 12 gauge's capability as a waterfowl gun, the 3 1/2" shell was introduced, which carries 1 3/8 ounces of steel shot. The 3 1/2" 10 gauge magnum shell now carries up to 1 3/4 ounces of steel shot. The 3" 20 gauge shell carries up to 1 ounce of steel shot.

Modern field and target loads for the various shells are generally loaded with lead shot as follows. The field load for the 10 gauge, which used to be loaded in a 2 7/8" shell, has been discontinued, although a 3 1/2" shell containing 2 ounces of shot is still offered as a turkey load. The standard 12 gauge field and target load is a 2 3/4" shell loaded with 1 1/8 ounces of shot. The dying 16 gauge is still offered in a 2 3/4" shell with 1 ounce of shot. The 20 gauge field and target load is a 2 3/4" shell with 7/8 ounce of shot. The 28 gauge target and field load is a 2 3/4" shell with 3/4 ounce of shot. The little .410 is available with a 1/2 ounce target and field load in a 2 1/2" shell or an 11/16 ounce field load in a 3" shell.

The latest trends in waterfowl shells seems to be the use of non-toxic shot made from denser (and therefore ballistically superior to steel) materials. Tungsten-nickel-iron is one such alloy from Remington, and Federal offers tungsten-iron. Steel shot is hard on thin wall barrels (such as are used in most double guns), so non-toxic shot that is safe for such barrels has been developed. Tungsten-polymer shot has been developed by Federal to solve this problem, and the Bismuth Cartridge Company offers shells loaded with bismuth shot. Non-toxic shot shells are also being produced for target and even upland game hunting, although there is no rational reason for this.

The following series of articles examines each of the surviving shotgun gauges in greater detail. A little bit of history and a lot more information about specific loads and their applications are included in those articles.