Shotgun ammunition is measured in gauge rather than in caliber and because shotguns are very versatile firearms, ammunition manufactured for them comes in a very wide variety of types and varying sizes and power. The gauge number is on the shotgun ammunition box. Your shotgun ammunition much match the gauge of your shotgun.
The term “gauge” describes bore diameter, but unlike “caliber” used for handguns and rifles, the larger the number, the smaller the bore (The inside diameter of the barrel). So a higher gauge number means the internal diameter of the barrel is smaller, while a smaller gauge number means the internal diameter of the barrel is bigger, so a 12-gauge bore is bigger than a 20-gauge bore. Shotgun ammunition is measured in gauge as well, rather than in caliber for handguns and rifles.
Gauge was defined by the number of solid balls the same diameter as the inside of the barrel that could be made from a pound of lead. Thus, the 10-gauge shotgun is larger than the 12-gauge, which is larger than the 20-gauge. While there are many different sizes, or gauges, of shotguns, the two most often recommended for home defense are 12 gauge and 20 gauge shotgun.
The length of the shell is another very important number. Not all lengths will feed in all shotguns. The common lengths are 2-3/4 inches, 3 inches, and 3-1/2 inches. The longer the shell, the more shot pellets and powder it can contain. Shotguns which are designed to load a shorter shell should never be used to fire a larger shell, even if the larger shell physically fits within the gun.This can be extremely dangerous as the gun may not be able to handle the higher pressures a more powerful ammunition produces.
There are three basic types of shells:
Dram Equivalent will tell you how powerful the ammunition is. Originally, drams were a black powder weight measure, but now modern shotgun ammunition uses smokeless powder. Shotgun ammunition manufacturers use dram equivalents to indicate how much power the load has. The higher the dram equivalent number, the more energy the ammunition has and the faster the shot will travel.
Shotgun ammunition falls into three general categories:
BIRDSHOT – Shotgun ammunition which uses very small pellets with individual projectiles of less than .24″ in diameter are designed to be discharged in quantity from the shotgun. The size of the shot is given as a number or letter–with the larger number the smaller the shot size. It is so named because it is most often used for hunting birds. The finest size generally used is #9 which is approximately .08″ in diameter and the largest common size is #2 which is approximately .15″
BUCKSHOT – A type of shotgun ammunition that uses medium-sized to large-sized pellets of .24″ in diameter or greater, designed to be discharged in quantity from a shotgun. Generally the larger the pellets, the fewer of them in the casing.
SHOTGUN SLUG – An individual cylindrical projectile designed to be discharged from a shotgun. As a single projectile, slugs must be carefully aimed to be effective.
Buckshot is generally recommended for home defense. The larger the buckshot the greater the stopping power and the greater the chances of over-penetration and injury to innocents in other rooms or buildings. If you are in a densely populated home or neighborhood you can minimize the risk of over-penetration by using small game loads of #6 Birdshot or smaller, but this sacrifices a great deal of effectiveness. Versatility in available loadings is a great strength of shotguns, but you must choose your loads carefully for your environment.
If you have a typical defense shotgun firearm with an 18″-20″ open-choked cylinder barrel, the pellets will spread out about 1″ for every yard of range.