The cartridge case

The cartridge case has two major functions. First, it holds together all the active components of the cartridge - the projectile, propellant and primer -ina waterproof container which is rugged enough to withstand rough handling, especially in automatic weapons. Second, when the gun is fired, the cartridge case is expanded by the pressure against the walls of the firing chamber, forming a gas-tight seal which prevents any propellant gas from seeping back into the gun mechanism - and possibly into the firer's face.

The second function explains why brass is still the most popular material for small-arms cartridge cases, despite its weight and cost. Standard cartridge brass (70% copper, 30% zinc) has exactly the right characteristics in that it expands instantly to form a seal without being split open. Nowadays steel is normally used in larger cannon, particularly linear-action guns with a high rate of fire, because its extra strength is better at coping with the violent treatment the cartridge is given by the loading mechanism. It is also slightly lighter and cheaper. However, it requires more protection against corrosion and, being less resilient, forms a less perfect seal on firing. This is a potential disadvantage in turret-mounted applications as it means that more propellant gas might seep back into the turret.

Light alloy has even more attractions (it saves 25-38% of the total weight of a cartridge) but for a longtime it proved very difficult to achieve satisfactory results with this material and it has only recently come into common use. The first and most notable example is the 30 X 173 ammunition for the General Electric GAU-8/A fitted to the A-10 aircraft. This has such a large ammunition capacity that a weight saving of over 270kg is achieved compared with the Oerlikon KCAs dimensionally similar steel-cased rounds (the actual difference in empty case weight being 143g instead of 350g). Light-alloy ammunition in this calibre is also used in the Mauser MK30 (the weight of a complete HE/I cartridge being 670g compared with 890g for the steel-cased rounds) and has been developed for the 30 X 113B used in the M230 chain gun.

The earliest cartridge cases were made in more than one piece, typically an iron head to which was fixed a tube made from coiled brass. These were very much an interim measure, soon replaced by the one-piece case which has been universal since the latter part of the nineteenth century. This is formed by discs, punched from a thick sheet of metal, being repeatedly "drawn* - that is, mechanically pressed into a hollow shape, until the desired case shape has been achieved. At the same time, information such as the cartridge designation, maker and year of manufacture are stamped into the head. The final operation is usually the cutting of the extractor groove close to the head.

20mm Machine Gun Brass Casing Bullet
Headstamp for 20mm Madsen, made by Kynoch in 1954 and loaded with an armour-piercing tracer projectile (not all headstamps are so helpful!)
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