Sizes in Use
Certain sizes are more common than others; 12 gauge is the most common size, with up to 50% of the overall shotgun market in the United States. The 20-gauge shotgun is popular with shooters who are uncomfortable with the weight and recoil of a 12 gauge gun, and is popular for upland game hunting. The next most popular size is the .410 shotgun, which is not actually a gauge but rather a caliber. 10, 16, and 28 gauges, while less common, are still readily available.
Shotguns larger than the 10 gauge are rarely manufactured nowadays. Eight gauge is rare in the United States due to its prohibition in duck hunting. However, it is still used in many parts of the world (notably: Britain) for bird hunting. Its shells are usually black powder paper cartridges as opposed to the plastic/wax cartridge and smokeless powder of today.
The 11, 14, and 18 gauge shells are the rarest of them all; people who own these types of rare shotguns will usually have their ammunition custom reloaded by a company that specializes in rare and custom bores for a high price. The very small 24 and 32 gauges are still produced and used in some European countries and Brazil. Punt guns and special purpose guns, such as the Russian 23 mm KS-23 (approximately 6 gauge), do exist, but are rarely encountered.
Also seen in limited numbers are smoothbore firearms in calibers smaller than .410, such as .22 Long Rifle, .22 WMR, and 9 mm rimfire, designed for short range pest control.
To further complicate matters, special shot cartridges are available for typical handgun chamberings such as 9 mm Parabellum, .45 ACP, .38 Special/.357 Magnum, .44 Special/.44 Magnum, and .45 Colt. These rounds are either crimped in or in a plastic casing replacing the bullet. These are not generally considered "shot shells" by shotgun users, and the patterning performance is questionable since they are fired through rifled barrels. Thompson/Center makes special pistol barrels in .38/.357, .44 and .45 Colt that have "straight rifled" chokes in them to reduce the spin of the shot column and produce better patterns, but they are still suitable only for pest control at very short ranges.
Read more about this topic: Gauge (bore Diameter)