Sunday, May 9, 2010

Cartridges: Centerfire cartridge

In the last post of this blog, we studied the rimfire cartridge. Now we will study another development: the centerfire cartridge (a.k.a the central-fire cartridge). Like the needle-gun cartridge, the pinfire and the rimfire cartridge, this cartridge also works on the same broad principle as the percussion lock (i.e.) use a hammer or firing pin to strike a blow on some shock-sensitive priming material (such as mercury fulminate or potassium chlorate) which then ignites and lights the main gunpowder charge. Since the case is metallic, it slightly expands due to the heat generated when the cartridge fires and thereby prevents gas from escaping from that side of the barrel.

The centerfire cartridge was invented by one Monsieur Clement Pottet of Paris and improvements were made by Monsieur Francois E. Schneider, also of Paris. The centerfire cartridge was introduced to England in 1861 by Mr. George H. Daw who owned a rifle manufacturing company and bought the English patent rights from Monsieur Schneider. However Mr. Daw quickly lost the patent rights in a case brought on by Messrs. Eley Brothers, a rival manufacturer of cartridges and small revolvers, owing to the fact that the patent was not kept in force in France, where the invention was originally protected. Centerfire cartridges began to quickly replace rimfire cartridges in many applications where higher pressures were required. In fact, centerfire cartridges are the most common cartridge type in use today and are used in most modern weapons currently.

In a centerfire cartridge, there is a small copper or brass cap in the center of the cartridge, which contains a shock-sensitive priming material. The rest of the cartridge case is filled with gunpowder and there is a bullet attached to the other end.

To fire this, one must strike the primer cap with a sharp blow. This detonates the shock-sensitive primer which ignites the gunpowder and thereby discharges the bullet.

In a typical centerfire weapon, a spring-loaded hammer is pulled back against spring pressure. When the user pulls the trigger, the hammer forces a firing pin to forcefully strike the primer cap and discharges the weapon. The firing pin is aligned to strike the cartridge at the center of its base, where the primer cap is located.

There are two forms of primers that are in use today. The one that is commonly used in many European (and Asian) made cartridges is the Berdan and the one used in American made cartridges is the Boxer. Ironically, the Berdan system was invented by Hiram Berdan, an American, while the Boxer system was invented by Colonel Boxer, an Englishman! Note that centerfire weapons can fire cartridges using either the Berdan or the Boxer system, as long as the cartridge sizes are the same.

In the Berdan system (patented in the US, March 20th 1866), the cartridge has a small bump in the bottom of the case, with a couple of vent holes. This is called the "anvil" of the cartridge. The priming material is placed in a small metal cup that is placed on top on the anvil and sits flush with the base of the cartridge. The hammer or firing pin hits the metal cup, which crushes the primer into the anvil and detonates it. The vent holes in the anvil allow the flash of the primer to reach the inside of the case and ignites the gunpowder. The Berdan system allows reloading the cartridge, but needs some specialized tools and a bit of patience. The used primer must be removed, usually using hydraulic pressure or a special tool, to remove the primer from the bottom. Then a new metal cap containing primer is carefully positioned in the bottom of the cartridge and the rest is filled with gunpowder and bullet. The Berdan primer is hard to remove from the cartridge case without damaging the anvil. This is why it is used by many European (especially Eastern-bloc) and Asian manufacturers to discourage reloading of cartridges. However, many a guerrilla has patiently reloaded Berdan cartridges using a match head and a piece of tin for the primer.

In the Boxer system (patented in the UK on Oct. 20th 1866 and in the US on June 29th 1869), the principle is similar to the Berdan system, except that the anvil is not part of the cartridge case. Instead the anvil is a separate part that sits on top of the primer cup and it has a large hole in the center. This makes it vastly simpler to reload the cartridge, since the primer can be easily removed by pushing a thin metal rod into the hole and pulling out the anvil and used primer. A new priming cup with the anvil on top is then pushed into the base of the case. Then the gunpowder and bullet are added and the cartridge is ready for re-use. Most manufacturers sell the anvil and primer cap as one part, even though they are really two different parts. Cartridges using the Boxer system were originally more complex to manufacture, due to the fact that the primer has two parts (anvil and priming cap). However, automated machine manufacturing techniques have completely removed this issue and the cartridge itself is simpler to manufacture even if the priming system is more complex. Since reloading these cartridges is much easier, this system became more popular in the United States, almost completely replacing the Berdan system. The United States Government does not discourage cartridge reloading and a user can save over 80% of the cost of a cartridge by reloading it.

It is impossible to tell whether a centerfire cartridge is using the Berdan system or the Boxer system of priming just by looking at it. The difference can only be told after the cartridge is fired, because one can look at the base of the cartridge case and see if it has two smaller vent holes (Berdan system) or a larger single hole (Boxer system). The same weapon can fire cartridges using either system.

Military ammunition from European and Asian manufacturers usually use the Berdan system because it is cannot be reloaded easily and is slightly cheaper to manufacture. They also often use corrosive compounds for the primer, in order to keep costs even lower. This means that the weapon needs to be cleaned after firing, to prevent the corrosive materials from affecting the firing mechanism. In contrast, modern American-made cartridges often use non-corrosive primer caps, though older ammunition still used corrosive primers.

  1. User can speedily reload ammunition. Much more convenient than the ammunition used from matchlock days.
  2. Ammunition is not affected that much by weather and can be stored for a long time.
  3. The metal cartridge expands slightly due to the heat generated when the cartridge fires. As a result, this provides a very tight seal and gas does not escape from the back of the weapon as much. This makes the cartridge highly efficient for its size.
  4. Cartridge doesn't need to be specially positioned in the weapon, unlike a pinfire cartridge. This makes it faster to reload.
  5. Can also be used with high-powered weapons, unlike a rimfire cartridge which can only be used with low-powered weapons. This is because it can be made with a thicker case, unlike a rimfire cartridge which requires thinner metal around its rim by its nature.
  6. Cartridges can also be easily refilled (at least those using Boxer primers), unlike a rimfire cartridge. Bullet, gunpowder and primer are three separate components and can be easily separated in a centerfire cartridge. By contrast, rimfire cartridges are deformed at the rim upon firing and need replacement of the entire case.
Due to its general purpose use and overwhelming advantages, center-fire cartridges form the bulk of modern ammunition for small-arms today.

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