The origin of the definition of the gauges for smooth-bore weapons is attributed to the British who called them in relation to the number of spherical balls of the same diameter can be obtained from a pound of lead. For this reason, the smaller the gauge, the greater the number that identifies it. In the age of front loader firearms, there was no real definition of the gauge because black powder and lead were paid into the barrel and taking shape and volume dates from the tube.
Turning to the creation of the cartridges, it was necessary to define standards so that all the ammunition produced for a shotgun had the same section. But there were many states with different measure units so in Liege, in 1911, were defined dimensions and related tolerances. This is true for the chamber of the cartridge that can take inside equal size cartridges, but the gauge of the barrel may vary so it can shoot balls also very different from each other. Let’s take an example. The Convention C.I.P. states that the chamber of cartridge can vary between 18.2 mm and 18.6 mm in gauge 16, and the chamber between 20.3 mm and 20.4 mm in gauge 12.
The work of the hunting shotgun
The shotgun has the purpose to perform a job using the energy produced by the combustion of gunpowder. Part of this energy is absorbed by the passive resistance of the barrel, another part is converted into heat and another part generates the thrust of the hunting shots. This last part is defined as “useful work” of shotgun. The energy called “kinetic energy” that animates the shots will be greater as much as greater will be the “useful work”.
The useful work of the shotgun is made up of three components: average pressure in the barrel, section of the barrel, length of the barrel. If we consider a length of fixed barrel, if the section is reduced, and therefore the gauge, increases the operating pressure. But this pressure can not increase without limit both for safety reasons, but also because it would distort the hunting shots that should lose their spherical shape. For this reason the weight of the charge that a rifle can launch remains linked to the diameter of the barrel, and then to the gauge. In the final analysis it is therefore possible to say that the smooth-bore hunting shot can shoot heavier loads if the caliber is large or, and this is very important, shotguns at the same weight charge develops lower pressures.
Traditionally gauges of shotguns are divided into 4 categories:
1) BIG GAUGES – (4 – 8 – 10) – In practice no longer used (except the 10 in some countries) because are impractical to use and designed to shoot too heavy loads.
2) AVERAGE GAUGES – (12 – 16-20) – Used a lot around the world. They are to be considered the best compromise for all the small game. The gauge 12 is the most used, but the use of gauge 20 is also good. The gauge 16 was not used and was almost disappeared until it is resumed in some countries. In Italy there is a very active Club (www.calibro16.it) bringing it back to its former splendor.
3) SMALL GAUGES – (24-28 – 32-36) – Used in Europe for small birds, they have seen a rapid reduction in their use almost unti to disappear (24-32). Others gauges, instead, in recent years, have had a second life (28-36) also to be used on the fields of skeet shooting. In particular, the gauge 36, in its magnum version (ie. 410) has always had a wide use in the United States and is having an interesting use even in some European countries.
4) MINIMUM GAUGES – (9 mm, 8 mm, 6 mm) – Not used to hunt for their low ability to break down the game.
The prohibition in the USA and in most countries of Europe of the big guns allowed to develop in the 12-gauge and 20-gauge the Magnum concept. These have the ability to shoot effectively both standard charges of shooting shots that oversize that arrive at the bigger gauge. This development was made possible thanks to modern steel manufacturing industry and to hunting powder production.