DDischarge of a Firearm

The firing mechanism of a firearm consists of a mechanical device which causes a hammer to fly forward and deliver a blow to the firing pin when the trigger is pulled. In some firearms the hammer and firing pin are made in one piece. The firing pin goes through a small hole in the breech face and strikes the primer cup. [The well-known phrase "lock, stock and barrel" originates from firearms—the lock is the mechanism of the firearm, the barrel is the tube through which the bullet travels, and the stock is the means of holding the firearm (butt/handle/grip)].

The primer cup contains a mixture of chemicals which sensitize each other to percussion and rapid burning, and consequently the primer burns rapidly producing a flame and a shower of hot particles that penetrates and ignites the propellant.

The burning of the propellant very rapidly produces a large volume of gases in a confined space accompanied by a substantial temperature and pressure rise. The resultant gas pressure forces the bullet away from the cartridge case and down the barrel of the firearm. The temperature and pressure rise also serves to cause the cartridge case to expand in the chamber, thereby effectively sealing the chamber to prevent any rearward escape of gas (obturation), which would lead to a reduction in pressure and consequently a reduction in bullet velocity.

The time span from the firing pin hitting the primer cup to the bullet leaving the gun is typically in the region of 0.01 to 0.03 seconds.4 Muzzle velocities range from approximately 600 feet per second for very low power handguns to approximately 3,500 feet per second for very powerful rifles. Temperatures and pressures inside a gun during discharge can be in the region of 3,000°C5 and 50,000 pounds per square inch.6

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