Before we start, the reader is advised to revisit an earlier article on bore/gauge of a shotgun, which will come in useful to understand some concepts discussed in this article. The reader may also find it useful to peruse the article on shotgun pattern testing.
The first, and most common type, is the shotshell. This is a cartridge that contains a number of pellets, usually made of lead, steel alloy, bismuth alloy or titanium composite. The most common type of shotshell is birdshot, which is commonly used to hunt birds. It consists of a cartridge containing dozens to hundreds of small pellets or ball-bearnings.
Public domain image
The shotgun shell is cylindrically shaped. In earlier days, the outer case was made of brass or thick paper. Modern shotgun shells are usually made of plastic with a thin hollow brass base cover, such as the illustration above. From left to right, we have the brass base of the cartridge, the propellant material (the gray part) which sits inside the brass base and extends out of it, a lot of wadding (the olive, pink and brown bands), the shot pellets and finally, another wad (the brown band on the right) that holds the shot pellets in the cartridge. The purpose of the olive, pink and brown wad is two-fold. First, it provides a gas seal between the pellets and the propellant. If it is not there, the propellant gas will simply flow through the gaps between the pellets instead of propelling the pellets. The second reason is that the wadding acts as a shock absorber or a cushion. When the shell is fired, the wadding gets crushed first and absorbs some of the shock. Without it, many pellets could get deformed by the propulsive force and thereby fly in the air erratically. The cushioning provided by the wadding prevents this.
The pellets in a shotgun shell are of uniform size. In earlier days, the pellets were mostly made of lead, using a process we described previously, because lead is cheap, easily formed and widely available. However, lead is a poisonous substance and can cause lead poisoning (e.g.) pellets that fall into ponds when hunters are hunting water birds. Water fowl could accidentally ingest some of these pellets and end up poisoned. Then, these could be eaten by birds of prey or animals and in turn, they get poisoned as well. People drinking the water could get affected too. These days, the environmental effects of lead are being taken more seriously and therefore, lead pellets are banned in several areas. Hence, modern pellets are now made of steel, bismuth or titanium composites.
Birdshot, like the name suggests, is used to hunt birds. Different gauges are used depending on the species of birds being hunted.
The next variation of shotgun shells use buckshot. These are similar to birdshot, except that the pellets are larger in diameter. Buckshot is designed to take down larger game animals, such as deer (which is why it got the name "buck" shot).
The next common variant is the solid slug. This is a single, solid, heavy bullet used to hunt large game. Many slugs already have rifling cut into them. The first design of a solid slug was from a German designer called Wilhelm Brenneke in 1898. His design has remained largely unchanged until now.
A Brenneke slug. Public domain image
The above image is of a Brenneke type slug. Notice the grooves cut into the sides of the slug. As before, there's a large amount of wadding (the white and brown parts) between the propellant and the slug. This kind is very popular in Europe and sometimes referred to as "European type" slug.
Another design is the Foster slug, designed by an American named Karl Foster in 1931. In this type of slug, there is a deep hollow inside, so that the center of mass is closer to the tip of the slug. This is designed for smoothbore barrels and because of the position of the center of mass, it doesn't tumble in the air because the drag causes the back of the slug to stay behind the front (much like a shuttlecock's feathers). This type is popular in the US and is sometimes referred to as the "American slug". It is also possible to fire this type of slug through a rifled barrel, but it causes lead buildup in the rifling grooves to happen at a faster rate.
Yet another type is the sabot slug. The word "sabot" comes from French, where it was used to describe a type of shoe worn by many workers during the industrial age. Incidentally, there was a period where the French workers went on strike, protesting their work conditions, and threw their sabot shoes into the industrial machines, hoping to jam them up. This is the origin of the word "sabotage"! In a sabot slug, the slug is usually an aerodynamic shape that is smaller than the barrel, surrounded by an outer shoe (the sabot) that provides a tight gas seal. The sabot gets deformed by the propellant pressures, while the slug inside is largely undeformed and intact. In case of rifled barrels, the sabot also has the rifling and therefore provides spin to the bullet. Once the bullet clears the barrel, the sabot separates from the bullet and falls down and the undeformed bullet continues on its way. This provides for better accuracy and faster velocity of the bullet. On the flip side, sabot slugs are more expensive and more time-consuming to manufacture.
The next type of shotgun ammunition is the bean bag round, also known by its trademark, flexible baton round. It consists of a small fabric container filled with birdshot. It is designed to be "less lethal" than the rounds we've studied so far. This type of round is designed to stun a person rather than kill them. Of course, fired at close range, it can be lethal as well. In longer ranges, it could be lethal if it strikes a vulnerable part of a person's anatomy, such as the throat or solar plexus. This type is typically used by police to incapacitate and capture a suspect. Other variations use rubber shot instead of bean bags for the same effect.
Another older form of the bean bag round was the rock salt shell. As the name suggests, shells were filled with rock salt crystals. Since salt crystals are brittle, these shells were not as lethal at longer ranges, but still caused a stinging bruise, enough to dissuade a person from attacking. These were used by police and rural farmers in earlier days.
Another type of shotgun shell is used for riot control. This is the gas shell and it normally contains pepper gas or tear gas.
Breaching rounds are often seen in use with police and military units. This is a special round designed to destroy door locks and door hinges, without endangering nearby bystanders. These rounds are also called Disintegrator or Hatton rounds. The slug is made of a dense metal powder, which is held together by a binder material such as wax. When fired at a door lock from close ranges, the slug destroys the lock and quickly disintegrates into a metal powder, instead of ricocheting somewhere or penetrating through the door. This is what makes is suitable to be used in tight confined spaces.
There are also other ammunition types, such as those that provide a lot of flash, or a whistling noise. These are used to scare or disorient animals, but are not lethal.
As you can see, there are many types of ammunition for shotguns, all for different uses.