In the case of shotguns, different types of cartridges are used. In some cases, these contain a single large ball, in which case the previous methods of manufacture were used. However, certain types of shotgun cartridges use dozens of small spherical pellets instead of a single lead ball. In this case, manufacturing these small pellets using the casting techniques from the previous post is simply not practical. Instead, a different technique is used to manufacture these pellets. This technique is usually used to manufacture pellets up to 6 mm. in diameter or so, as it is not suitable for larger diameters.
We will first study the historical technique, which was invented in Bristol by one William Watts in 1782. The process starts by acquiring a location where lead may be dropped from a height of 40-60 meters. This could be a tower on the ground (called a shot tower), or an old mineshaft. In William Watts' case, he had a natural cave running under his house, so he built a three story tower and then dug a shaft in the ground till he hit the caves. There is a furnace at the top where lead can be melted. The molten lead is poured into a pan that has holes in the bottom of it, corresponding to the diameter of the pellets desired. The molten lead slowly percolates through these holes and forms globules which fall down to the bottom of the tower or mine shaft. During their fall, the molten lead globules become spherical shaped, much the same way as raindrops form spheres as they are falling so that surface tension is minimized. During their fall, the globules also harden in the air. At the bottom of the shot tower or mine shaft is a container filled with water. The fall through the air must be long enough for the pellets to harden sufficiently before they contact the water container, otherwise they will be flattened on impact. Typically, the pellets are dropped from a height of 40-60 meters for this to happen, though some shot towers are even taller. For instance, the Phoenix shot tower in Baltimore, MD, was the tallest building in the US at 234.25 feet (71 meters), when it was first built in 1828. The lead must not contain any zinc impurities and must have a small amount of arsenic in it, in order for the globules to form properly.
The more modern technique is the Bliemiester method invented by Louis A. Bliemiester of Los Angeles, CA and has been in use in the US since 1959. In this method, molten lead is dropped for a short distance of about 1 inch (approx. 25 mm.) into a container of very hot water. The pellets then roll down an underwater incline and then drop another 3 feet (approx. 1 meter). The hot water controls the rate that the lead pellets cool and harden and the surface tension ensures that the pellets are spherical. This method does not require a tall shot tower to be built and hence is the preferred method these days, while shot towers are now largely historical landmarks.
After the lead has cooled down, the shot pellets are gathered from the water container and classified into different shot sizes and any imperfectly shaped ones are removed and remelted. In the Victorian era, this was done manually by women who would gather the pellets in their aprons, dry them and judge whether each pellet was properly formed on not, by looking at them. The modern process is automated and is consequently faster. In the modern process, the shot pellets are taken out of the water container and put into a steam-jacketed tumbling barrel, where they are dried and polished. Next, they are taken to grading tables. These grading tables are located on an inclined slope and consist of a series of "steps" with gaps in between them. Each step is slightly lower than the previous one. The shot pellets are allowed to roll down the tables. Pellets that are sufficiently spherical shaped will jump across the gaps between the steps. Pellets that are improperly shaped or have multiple pellets fused into one, will not jump the gaps and will fall through into scrap boxes. The bad pellets are collected from the scrap boxes and remelted. The good pellets are taken through a series of vibrating mesh screens, each of which have a mesh of a particular size. The mesh screens sort out the pellets into standardized sizes, by keeping pellets above a certain size on top of the mesh, while allowing smaller sized pellets to drop through them. These sorted pellets are then packed into bags and shipped out to a cartridge manufacturer.
If the pellets made with this technique are made of pure lead, then they are known as drop shot. If a small amount of antimony is added to the lead, then these pellets are known as chilled shot. Pure lead pellets are somewhat softer and thus they get deformed more when fired with more powerful cartridges. Addition of antimony to the lead makes it an alloy that is a bit harder and can withstand higher pressures.
As mentioned before, this technique can only be used for smaller diameters (less than 6 mm. or so) and larger shot must be manufactured using casting or swaging techniques. We have already studied casting in the previous post and we will study swaging in the next post.
The sorted pellets are used by cartridge manufacturers or home reloaders to manufacture cartridges.