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Overview of the structure of metallic centrefire ammunition

The cartridge case is the largest and often most expensive part of a cartridge, it contains the powder, primer and bullet. It is usually tubular in structure and mostly made of metal, closed at the base except for a primer pocket. The shape can be straight, tapered or bottle necked according to the calibre and design.

The cartridge mouth holds the bullet. The bullet is placed in the mouth of the cartridge and the mouth is then "crimped", or compressed onto the bullet. The mouth of bottleneck shaped cartridge cases is often referred to as the "neck".

  Straight, Taper and Bottleneck Case Types

There are six types of crimp, two of these are roll crimps.

The first sort of roll crimp is used when the bullet is inserted completely into# the case and the lip of the brass neck is rolled over with a very small radius onto a flat nose or the flange around the nose as occurs with "semi wadcutter" or "Keith" bullets.

  Roll and Bevel Revolver Crimps

The second type of roll crimp has the lip of the case mouth forced inwards into a cannelure or groove in the bullet.

Three others types are:- taper crimps, folded crimps and bevel crimps...

A taper crimp is when the lip of the case mouth is merely compressed against a bullet's sides.

  Roll and Taper Crimps

Bevel crimps occur on flat nose or semi wadcutters where the metal from the lip of the mouth is folded over the flange rather than rolled completely over.

Folded crimps are rare if used with a bullet, but are commonly used on blank and grenade launcher rounds. They are also used with shot loadings in metallic cases.

  Folded Brass Crimp

There is also a crimp type that is sometimes used on military ammunition. There are two distinct forms of this crimp, in both cases the bullet is seated to a greater depth so that the cannelure (if present) is well below the lip of the case mouth. In one case the crimping dies (usually four) form a ring where the cannelure in the bullet is, forcing case metal into the cannelure. In the other style the dies are slightly oblique to the axis of the cartridge (at a similar angle to the rifling in the barrel) and they press case metal into shallow depressions that are formed in the bullet jacket by the crimping action. This heavy crimping serves two purposes, it ensures that the bullet remains in place during rough handling and it enables the initial pressure to rise to a sufficient level for reliable and consistent firing.

  Military Crimps

Roll and bevel crimps are commonly employed in revolvers due to high recoil that could dislodge a loose fitting bullet and jam the cylinder. Automatics have less recoil and thus can "get away" with the less positive grip of the taper crimp, but often use the case mouth as the location for headspacing the cartridge.

The head of the case has several features.

The flat base has a recessed central area where the primer is fitted, around this there is an annular groove which is formed due to the curvature of the outer edge of the primer and the "lead in" curvature of the primer recess or primer pocket. There is sometimes a second "crimp groove" outside of this, that is intended to increase primer retention. The two annular grooves are often confused with each other. The outer margins of the base are usually impressed with various markings indicating manufacturer, batch, calibre and year of manufacture. These markings are collectively known as the headstamp.

  typical case base

There are seven types of head which are:- rimmed, rimless, rebated, belted, belted rimless, rebated belted rimless and countersunk.

A rimmed case has a flange around the head that is larger than the case body. Often used on a cartridge designed for revolvers or older rifles.

A rimless cartridge case has a "V" shaped groove around it's head and is the same diameter as the back end of the case. Often employed in self loading rifles and light machine guns.

A rebated rim cartridge is grooved in the same fashion as the rimless, but the grooved portion has a smaller diameter than the rest of the cartridge. Like rimless cartridges, these cartridges are mainly designed for automatic weapons.

Belted heads have a narrow raised portion just in front of where a rim would be if it had one, this thickened belt has been used in some big game calibres as a strengthening measure, as a place to locate the round for correct headspace and in some pistols designed for use with "speed loaders".

Belted rimless is a combination of "belted" and "rimless" and is common on "magnum" calibres, where it helps withstand the stresses between base and case.

Rebated belted rimless is likewise a combination of "belted" and "rebated rimless".

  Case Bases... Rimmed, rimless, rebated, belted, belted rimless and rebated belted rimless

The seventh type is the countersunk head which allows the nose of the bolt to sit inside a recess in the base and extraction is by the internal lip within this recess. The external groove is not an extractor groove, but is caused by displacing case metal to form the internal 'mushroom' shape.

  Recessed or Countersunk Case Base

The primer pocket is a cylindrical recess in the centre of the base of the case. This is where the primer is inserted. The primer is sometimes retained by a ring shaped indentation that displaces metal into the annular groove. This is also known as a hard crimped primer. Identification paint is sometimes applied and then wiped off leaving coloured rings in this "crimp" groove and the annular ring.

Flash holes (1, 2 or 3 in number) are drilled in the base of the primer pocket. 1, 2 or 3 occur in Berdan primed cases, Boxer versions usually have just one large centre hole, but I have seen examples that had a central hole and two "D" shaped angled slots.

This page deals with ammunition that has a primer and primer pocket located in the center of its head and is thus called centrefire ammunition.

There is another method that can be used at low service pressures... This utilises a case that is made from sheet brass or steel that is drawn and swaged into shape. The shape incorporates a rim and there is an annular space within this rim that is filled with a priming compound that is initially liquid and flows into the rim then the compound dries and sets. The hammer or firing pin strikes the base of the case at the edge of this rim and the system is known as "rimfire". Mainly .22 calibre, but also used on some .44 calibre and .41 calibre "cowboy era" revolvers. Cartridge operated nail guns and captive bolt humane killers also use the rimfire system. The Number 3 garden gun and .22 dust shot cartridge (and some similar Flobert calibres) are the only shotgun cartridges to use rimfire ignition.

  Rimfire case base in cross section showing primer compound

The physical dimensions, the features of the case, the cartridge or rifle designers's name and the calibre it was designed for plus the calibre it actually uses (if it is a necked down version) are used to designate and identify a cartridge.

Propellant Powder or Gun, rifle, and pistol Powder form the powder charge which fills most of the volume of the case. This powder does not detonate or explode when ignited, but burns very rapidly and progressively, producing hot gases that cause high pressure in the case body. The high pressure gas pushes the bullet from the case mouth into the forcing cone, where it engages the rifling and progresses down the barrel of the gun. The pressure inside of the brass case, coupled with the intense heat, swells the case walls into close contact with the inner surface of the chamber, thus sealing the breech.

There are many different compositions of powders. They are sometimes described by the shape of their grains. There are, ball powders, disc powders, tubular powders, rod forms, extruded flake and flake powders. I have heard of a powder that was intended for large calibre revolvers that had a doughnut shape to the individual grains.

There are two main divisions within powders black powders and nitro powders. The nitro powders then fall into two types, single based and double based (dibasic).

Black powder is for older loads in old calibres and some blank cartridges.

Single base nitro cellulose powders are considered by some to be less stable at high ambient temperatures.

Dibasic powders contain nitro cellulose grains that have absorbed nitro glycerine, the result being a powder that is more stable than either of its constituents.

The primer sits in the primer pocket of the case head. It is composed of a soft copper, bronze or brass cup, a compound that will detonate due to percussion and sometimes an anvil to oppose the striker blow. When the primer is struck by a firing pin, the priming compound detonates sending an intense flame through the flash holes into the powder located inside the cartridge case. This flame then ignites many individual powder grains initiating the rapid burn which produces the hot, expanding gases that force a bullet along the barrel.

There are two major types of primer used today... the Berdan primer and the boxer primer.

Boxer primers are preferred in America, where they are considered to be more easily reloaded.

Berdan primers are simpler, cheaper and more reliable. These find favour in the UK and Europe where reloading does not rely on pushing out the spent primer with a pin, but uses a pulse of hydraulic pressure to expel the old cap.

The bullet is the projectile and the sealing method for the front of the case. Bullets may be cast from lead or lead alloy or have a lead core with a gilding metal (cupro nickel) jacket. The jacket may completely cover the bullet or only partially cover it. A third form exists that is a combination of the two, whereby a solid one piece bullet is cast or swaged from an alloy material that has physical properties similar to the overall properties of a jacketed bullet.

There are bullets under development that have moulded plastic jackets (as opposed to loose fitting sabots) or are made by injection moulding a plastic compound that has an admixture of heavy metal powder to increase the density. There is potential and scope here for a plastic moulded bullet with tungsten powder density improver that has a jacket or driving sleeve moulded from a plastic with good lubricating properties.

  Bullet core of moulded plastic or copper alloy with tungsten powder additive and injection moulded lubricating plastic driving jacket

Bullets are usually streamlined in shape to reduce air resistance and increase target penetration. There are exceptions and there are flat faced and various other forms of blunt nosed bullet designs.

Some bullets contain inclusions of tungsten alloy or hardened steel that increase the sectional density and/or provide armour piercing characteristics.

Bullet noses may have axial holes, hollow points or cavities to increase expansion on impact, various aluminium or plastic tip inserts may be used to modify the expansion.

Cast bullets often have several prominent grooves around the parallel portion. The grooves are used to retain large quantities of bullet lubricating compound that are needed to stop friction welding of lead particles to the internal surface of the barrel. These grooves also provide spaces for the prominent ridges to be deformed or swaged into by the rifling grooves. Jacketed bullets often have one (or more) rings of toothed indentations known as a cannelure, the main reason for this is bullet retention as there is no need to lubricate the jacketed bullets, indeed such lubrication would (could) give rise to a form of barrel damage known as "ring bulging".

Bullet bases can be flat or recessed and some use a tapered "boat tail" shape. Some bases are entirely covered by jacket metal and others have exposed lead. Cast bullets are sometimes (mainly) "gas checked" whereby a shallow cup of gilding metal is added to the base either during casting or pressed in position after casting. On tracer or incendiary bullets there is a central "match" compound that ignites the incendiary or tracer compound within the bullet from the flame of combustion of the main propellant charge.

Bullets come in many shapes and styles, each of which was generated with a particular purpose in mind. The multiplicity of shapes increases as each generation adds it's own fads and fancies to the melting pot. The different bullet shapes a headstamp variations have become a collecting cult in their own right.

 Revised... 18 July 2001, Revised... 04 September 2002, Revised... 09 October 2002, Revised... 30 December 2002, New Domain... 07 December 2003, Upgraded... 26 January 2007,
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