Elements of Ammunition
The Case
1. Body
The case is the part of a cartridge that holds the powder, primer and bullet. It is generally shaped like a vase, open at the mouth, hollow, and closed at the base except for a a small hole in the base. The case, especially when it is empty, is called brass.
The mouth of the cartridge holds the bullet. The bullet is placed in the mouth of the cartridge and the mouth is then crimped onto the bullet. There are two types of crimps, a roll crimp and a taper crimp. When a case mouth is pushed into a cannelure, it is called a roll crimp. When the case mouth is just pressed against a bullet body, it is called a taper crimp. They are usually utilized for revolvers and autoloaders respectively.
The cartridge may be shaped in a straight wall design, bottleneck design, or somewhere in between called a taper design. A straight wall design is clear enough; the walls of the case run straight up and down from the mouth to the base. A bottleneck design is similar in that most of the body runs straight up and down. Then, usually about two thirds from the base, the body slopes inward, narrowing the body. The walls will then abruptly begin running straight up and down from the sloping part of the shell to the mouth. This gives the cartridge the appearance of a wine bottle. A tapered shell usually looks like a straight walled shell, but the shell is narrower at the mouth than at the base. The M1-30 Carbine cartridge is an example of a tapered design.
2. Head
The base of the shell is called the head. It is of course circular. It is defined from the body of the case by one of two general features. It could be separated from the body of the case by a cannelure, or could lack a cannelure, but then have a distinctively larger diameter than the case body. A cannelure is a depression that runs around the circumference of the case body. A cannelure is usually produced on a shell body so that an autoloader’s extractor claw has a place to grab the shell body and pull it from the firing chamber.
There are three types of heads. They each produce three kinds of shell bodies. The three basic types of shell bodies are: rimmed, rimless, and rebated rimmed. A rimmed case has a head that is larger than the case body. One finds this most frequently on a cartridge designed for revolvers. A rimless shell case has a cannelure and a head that is of the same diameter as the shell case. One finds this most commonly on shells designed for autoloaders. Finally, a rebated rim cartridge has a cannelure and then a head that has a smaller diameter than the shell body. Like rimless cartridges, these cartridges are designed for autoloaders. They are not frequently encountered.
Each head has on its face an annular ring. An annular ring is a depression in the center of the case head. This is where the primer goes. The annular ring is commonly called a primer pocket. In the center of the primer pocket are one to two holes. These holes are called flash holes. When a shell uses a primer and primer pocket located in the center of its head, it is called a centerfire cartridge.
3. Dimensions
The defining characteristics of a cartridge are the case dimensions. These dimensions almost solely define a cartridge. The dimensions include the thickness of the case body at different locations, the interior and exterior case diameters, and the thickness and diameter of the head. When one orders a particular caliber or designation of ammunition, one is really referring in a shorthand to the many case dimensions that describe a particular shell.
An important measurement for a cartridge is its maximum overall cartridge length or maximum OCL. This is most important for cartridges designed for use in autoloading guns. The OCL is measured from the head to the tip of the bullet. Cartridges cases are often loaded with a wide variety of bullets, so the OCL may vary considerably by brand or loading. For a particular cartridge, there is a maximum cartridge length.
Powder
Gun powder, or powder, is loaded into the cartridge case. The powder is the propellant for a bullet. The powder does not detonate when ignited, but instead burns very rapidly. The act of burning produces hot gases that expand in the case body. These expanding gases push a bullet from the case mouth and launch it down the barrel of a gun. The pressure inside of the brass shell, coupled with the intense heat, swells the shell in the firing chamber, keeping gases from exiting the breech.
There are many different types of powders. They are usually described by the shape of their grains. There are, for instance, ball powders as well as extruded flake and flake powders. Various powders have unique burning rates and hence equal amounts of different powders, when ignited, may cause vastly different firing chamber pressures.
Primer
The primer is a small disk that is placed in the primer pocket or annular ring of the case head. It is composed of a soft copper shell like a tablet with wafers of explosive material inside. When a primer is struck, the wafers of explosive materials detonate and send flame through the flash holes of the case into the powder located inside the cartridge case. This then ignites the powder, causing it to burn and produce the hot expanding gases that speed a bullet on its way.
There are two major types of primers, Boxer and Berdan. Boxer primers are predominately used in America. These primers are self-contained units, holding all the necessary ingrediants and structures or their operation. The shells that receive these types of primers have a single flashhole in the center of the primer pocket. Berdan primers are missing the anvil, or the piece of metal that the detonation material is struck against. The anvil is part of the primer pockets of shells made to receive Berdan primers. Boxer primers are used on shells that are meant to be reloadable. A large pin is driven by a loading press into the shell through the case mouth. The pin pushes the spent primer out of the primer pocket through the single, large flashhole. Since the primer is fully self-contained, a whole new primer complete with anvil can be pressed into the empty primer pocket.
One of the big issues with primers concerns corrosive and noncorrosive primers. Corrosive primers use a detonating material that produces salts which can quickly rust a firearm. Generally, modern, factory ammunition uses only noncorrosive primers. However, shooters are always getting bargain ammunition to drive shooting costs down. A typical source of bargain ammunition is governments releasing their obsolete or excess ammunition on the American civilian markets. Militaries, including our own well into the 1950’s, made use of corrosive primers due to their perceived reliability.
Bullet
The bullet is the part final part of an item of fixed cartridge ammunition. It is composed most commonly of pure lead called dead soft lead or a lead alloy. It is usually tapered in some way so that it is thicker at its base than at its tip. Bullets usually have multiple cannelures that run around their bodies. These cannelures are not usually visible when the bullet is seated in the case mouth. The cannelures are inscribed on the part of the bullet that is inside the case. The cannelures are packed with grease to aid the bullet as it spins down a gun barrel.
Bullets come in many shapes, styles, and types, some of which will be described in further detail later in this work.
Setting the Standards
As we have seen, the case for a particular cartridge has a variety of statistics. The case, in a sense, actually defines the cartridge. And we are aware that when bullets and powders are mixed and matched to a case, a particular loading is developed. But are there upper and lower limits to loadings? After all, who is to say how much powder can be added to a cartridge without making the round so powerful it would destroy the gun its fired in?
There is an organization which defines the upper and lower limits of pressure that a cartridge can generate safely as well as other important characteristics. It is also the reference for the mechanical drawings of cartridge cases. It is the Sporting Arms and Ammunition Manufacturers’ Institute or SAAMI. SAAMI dates back to 1926. In a different form, it dates back to 1913. This organization, in conjunction with ammunition developers, sets the standards for what defines a particular cartridge, that is, its operating parameters such as maximum pressure it can withstand during firing, or maximum velocity a particular cartridge’s bullet should travel at, as well as all other vital statistics. When you buy a brand of shells that are of appropriate caliber for your gun, you know they will perform safely and accurately because the shell manufacturer and the gun manufacturer are all referencing SAAMI specs.
SAAMI specs allow a cartridge to be loaded, under the rubric of a certain bullet weight, with a variety of powders in order to achieve different velocities. Hence, an array of loadings are developed for the end user.
Of course, any set of specs can change suddenly. For instance, after World War Two, the specifications for the .45 Long Colt were changed by SAAMI, rendering older guns chambered for the cartridge essentially obsolete in the face of store-bought loadings. In the 1920’s SAAMI changed the specs for shotshells, making most shotguns up to that point mismatched with shells now offered by manufacturers.
Caliber