The cartridge case is designed to house the primer, propellant, and to securely retain the bullet in the neck of the case. Cartridge case design is affected by various factors, the most important being:
The role of the ammunition.
Type of weapon.
Design of the bullet to be used.
Type of ignition system, that is, Boxer primed or Berdan primed.
The vast majority of cartridge cases are made of brass (approximately 70% copper and 30% zinc) but other materials such as steel, coated with either zinc, brass, gilding metal, copper, lacquer or blackened; copper; nickel-plated brass; cupronickel (approximately 80% copper and 20% nickel); gilding metal (approximately 90% copper and 10% zinc); aluminum. Teflon-coated aluminum and plastic are also encountered.
Second in popularity to brass is steel. One specification for cartridge case steel is carbon 0.08% to 0.12%, copper 0.25%, manganese 0.6%, phosphorus 0.035%, sulfur 0.03%, and silicon 0.12%.39
Shotgun cartridges are usually plastic with a brass or coated steel base, but paper with a brass or coated steel base and all plastic shotgun cartridges are also encountered. "All brass" shotgun cartridges are also known in older ammunition, and are currently manufactured for reloading purposes. Some .410" caliber shotgun cartridges are all aluminum.
Apart from shotgun cartridges, brass is by far the most common material used for the manufacture of cartridge cases. Experience has proved brass to be the most suitable as it is strong, sufficiently ductile, nonrusting, suited to drawing operations during manufacture, of reasonable weight, and readily available.
The strict specifications and quality control procedures for cartridge manufacture reflect the very important role the cartridge case plays in the discharge process. In addition to housing all the components of a round of ammunition in one package, a cartridge must:
Be safe to store, transport, and use.
Seal against ingress of moisture and oil.
Consistently achieve the required ballistics performance even under very different climatic conditions.
Maintain performance after many years of storage.
Be sturdy enough to withstand rough treatment, especially on the battlefield.
Achieve moderate and consistent chamber pressures.
Function satisfactorily and reliably from belt- and magazine-fed firearms under sustained fire conditions.
Be relatively cheap and readily manufactured during periods of emergency such as war.
Retain the bullet for a period after ignition to allow the propelling gas pressure to build up sufficiently to achieve peak performance.
The type of brass used is very important to the manufacturing process and manufacturers carefully specify the quality of the brass to be used. Four examples of specifications are as follows40:
1. 68% to 74% copper and 32% to 26% zinc. Impurities must not exceed 0.2% nickel, 0.15% iron, 0.1% lead, 0.05% arsenic, 0.05% cadmium, 0.008% bismuth. Tin and antimony must be absent and there must not be more than a trace of any other impurity.
2. 65% to 68% copper and 35% to 32% zinc with up to 0.2% nickel. There must be no individual impurity in excess of 0.1% and no more than 0.1% lead, 0.05% iron, and 0.03% phosphorus.
3. 70% copper and 30% zinc with not more than 0.25% of all other impurities combined.
4. 72% to 74% copper and 28% to 26% zinc. Impurities must not exceed 0.1%, and there must not be more than 0.1% lead and 0.05% iron.
When a round of ammunition is discharged in a firearm, the internal gas pressure, and to a much lesser extent the temperature rise, causes the cartridge case to expand tightly against the chamber walls (obturation). This is an extremely important function of the cartridge case as this prevents the rearward escape of gas. Such an escape of gas would reduce the velocity of the projectile and consequently the efficiency of the firearm and could possibly cause a malfunction in the firearm mechanism.
If the brass in the cartridge case is too soft, it will not spring back from the chamber walls, which will probably make extraction of the spent cartridge case very difficult. If the brass is too hard, the cartridge case could crack because it is too brittle and jam the firearm mechanism. When the brass in the cartridge case is of the correct hardness, it springs back to its near original dimensions and the spent cartridge case is readily extracted.
For higher-velocity ammunition the hardness of the brass usually decreases from the base to the neck of the cartridge case. Cartridge cases for low-velocity ammunition are normally made to a standard hardness along their entire length.
The base of a cartridge case must be strong enough to withstand ramming and extraction (this can happen numerous times to an individual round of ammunition during loading and unloading procedures) while the neck of the case must be strong enough to rigidly support the bullet yet flexible enough to expand and seal the chamber during discharge. High-temperature discharge gases can raise the pressure inside the cartridge case to 40,000 pounds per square inch in a very short time period.41
As the cartridge case is subjected to considerable stresses during loading, firing, and extraction, case thickness as well as case hardness needs to be carefully controlled. There must be a sufficient thickness of metal at the base of a cartridge to sustain the severe back thrust that occurs during discharge. If the metal walls of the cartridge case are too thin, the extension of the cartridge case due to longitudinal stress may cause it to fracture or the wall to separate from the base. For these reasons the thickness of metal in a cartridge case is carefully controlled and decreases from the base to the neck. The need to keep the weight to a minimum is another factor that is taken into account at the design stage.
A large quantity of propellant is required for modern high-velocity ammunition and this is accommodated by enlarging the diameter of the case over most of its length before markedly reducing the diameter at the forward end to accept the bullet. High-velocity cartridge cases are tapered and necked to avoid extraction difficulties which would be experienced if cylindrical cases were used in firearms with high chamber pressures. Most low-velocity cartridge cases are also slightly tapered.
The feeding and extraction mechanism of the firearm coupled with the type of ignition system dictates the design of the base of the cartridge case. Nearly all cartridge cases have the outside surface of the base indent-stamped by the maker (head stamp). Information such as the maker's initials, code, or mark, year of manufacture (mainly military ammunition), caliber or other coded information are indent-stamped into the base. It is sometimes possible, even for old ammunition, for a manufacturer to check its records and give the complete specification of a round of ammunition from the head stamp details.
The joint between the primer cup and the outside of the cartridge case base is sealed with lacquer to prevent the ingress of moisture and oil. The lacquer is sometimes color coded as an aid to visual inspection during manufacture, and also sometimes to identify the type of bullet, for example, ball, tracer, armor piercing. Sometimes the mouth of the case is internally varnished, just before inserting the bullet, to waterproof the joint and to provide resistance to the pressure of the propellant gases. (Hornby has developed a special black nickel plating for use on all metallic cartridge cases. It is claimed to give a smoother surface than conventional ammunition leading to greater reliability with fewer weapon malfunctions.)
Bogus head stamps are sometimes encountered when a government, for political or economic reasons, is supporting a rebel cause in another country by supplying the rebels with ammunition. For obvious reasons the source of supply is disguised. This can be done by omitting the head stamp or by using fake head stamps.42 It is not unknown for such ammunition to be head-stamped in such a way as to attempt to place the blame for supply on some other government.
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