Tuesday, April 20, 2010

Percussion Lock or Caplock Mechanism

So far, we've covered firing mechanisms all the way to the flintlock. The flintlock was a major advancement in firing technology and it stayed active for over one hundred years. One of the disadvantages of it (and all previous technologies, the matchlock, snaplock, wheel-lock etc.) is that when firing the weapon, the basic principle in all these weapons is to light some gunpowder in a firing pan which is attached to the barrel, but located outside the barrel and have the pan connected to the main gunpowder charge inside the barrel by a touch hole. The flame first lights in the firing pan and then travels through the touch hole and lights the main charge of gunpowder, which then ejects the bullet. Because of this, there is a noticeable delay from when the flame lights in the pan to when the gun actually fires.

The next big advance was due to a Scottish clergyman, one Rev. Alexander Forsyth. The good clergyman was also an avid hunter and liked to go bird hunting in the marshes. While hunting, he noticed that the birds would often spot the flame from the pan and immediately change direction. Since there is a delay between when the contents of the pan light up and when the gun discharges, this delay was long enough for many birds to escape. Rev. Forsyth made several experiments and finally settled on using mercury fulminate as his ignition mechanism. His patent application (granted April 11th 1807) reads as follows:

"I do make use of some one of the compounds of combustible matter, such as sulphur or sulphur and charcoal, with an oxymuriatic salt; for example, the salt formed of delphlogisticated marine acid and potash (oxymuriatic of potassium), or of fulminating metallic compounds, as fulminate of mercury or of common gunpowder, mixed in due quantity with any of the aforementioned substances, or with an oxymuriatic salt as aforesaid."

The specification is pretty broad and doesn't reveal too much, so we'll study what this means. First, we determine what a fulminate is. Ordinary black gunpowder and some other explosive materials have the property that they may be ignited by striking them with some force between two metal faces. However, the resulting explosion doesn't provide any more force than if they were lit with a flame. A fulminating substance is one that is reliably ignited by percussive force and the resulting explosion is more energetic than if it were lit by a flame or by any other means. The most well-known fulminate is potassium chlorate. Some readers might have played with a roll-cap toy pistol as kids, where the tiny caps explode when struck under pressure. There are other fulminates such as mercury fulminate, silver fulminate, gold fulminate etc.

The earliest research into these substances was made by a Frenchman named Peter Bolduc, prior to 1700. There were several reports published between 1712 and 1714 by the Royal Academy of Sciences, of experiments by Nicholas Lemery. Bayen, the chief physician of King Louis XV discovered mercury fulminate in 1774, Fourcroy studied them in 1785, Vauquelin in 1787, and Berthollet discovered silver fulminate in 1788. Until then, no one thought of using fulminates to firearms. The first few experiments by Berthollet ended in failure because the material was deemed too sensitive and he finally gave up after a couple of close calls. An Englishman named Howard invented a new priming gunpowder in 1800 using the research of Fourcroy and Vauquelin, but it was also not too popular. Finally, it was left to Rev. Forsyth to make the necessary discoveries and get the patent. In his patent application, he also reveals his firing mechanism:

"Instead of permitting the touch-hole, or vent, of the species of artillery, fire-arms, mines etc. to communicate with the open air, and instead of giving fire by a lighted match, or flint or steel, or by any other matter in a state of actual combustion, applied to a priming in an open pan, I do so close the touch-hole or vent by means of a plug or a sliding-piece as to exclude the open air, and to prevent an sensible escape of the blast, or explosive gas or vapour, outwards, or from the priming or charge; and, as much as it is possible, to force the said priming to go in the direction of the charge, and to set fire to the same, and not to be wasted in the open air."

The basic idea of his patent ran like this:

Public domain image courtesy of wikipedia
It consists of a small metal nipple sticking out in the back of the barrel, with a hole that leads into the barrel. The main charge of gunpowder is loaded into the barrel. To the nipple is placed a small copper cap containing a percussion sensitive explosive such as mercury fulminate or potassium chlorate. The hammer is pulled back against spring pressure. When the trigger is pulled, the hammer is released and hits the copper cap covering the nipple with sufficient force to detonate the mercury fulminate (or potassium chlorate or whatever). The flame from the resulting explosion travels into the barrel and ignites the main gunpowder charge.

With this invention, there is very little delay from when the trigger is pulled to when the gun discharges. It is also not affected by weather and was more reliable than some of the previous systems. While Mr. Forsyth did get his patent and win a significant case of patent infringement in 1819 (Forsyth vs. Reveire), he did not pursue his invention very further and went back to his pastoral duties in his church. As a result, some manufacturers found creative ways to evade his patent and others waited for his patent to expire before making weapons that used this system.

Future firing mechanisms such as cartridges also use this same basic idea (i.e.) use a tiny amount of pressure sensitive explosive to detonate the main charge of explosive. In fact, this principle is still used in the majority of weapons to this present day.

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