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Know your gun

The 'bang' you hear is when the shot, wad and hot gas exit through the barrel into the open air.

Tuesday, 26 June 2007

'Lock, stock and barrel' is a common enough phrase in the English language, signifying completeness in every respect!

IT IS AN expression taken from gun making, and a lock, a stock and a barrel have been the essential elements of a gun from the earliest times.

The meaning of barrel is obvious, and most folks know the stock is the wooden bit that goes against your shoulder. The lock is the mechanism that makes the gun fire when you pull the trigger.

To understand the sequence of events when your gun fires, it's best to start by looking at a cartridge. Inside the tube, at the front, is the charge of shot. Behind that is a wad, which pushes the shot through the barrel and forms a gas seal. Behind that is the propellant charge - a chemical compound called nitro-cellulose. Like all propellants and explosives, nitro-cellulose generates its own oxygen as it burns, and can therefore burn in a vacuum.

Behind the propellant charge is the primer. That's the little copper-coloured disc you can see in the middle of the brass head. The primer is a little, soft metal cup containing a relatively unstable chemical compound which bursts into flames when hit.

The actual firing sequence begins when you open your gun to load it. The opening of the gun forces internal hammers back against springs, and when you close the gun the hammers are held in the fully back position by catches known as sears. These sears are attached to the trigger mechanism.

When you pull the trigger, a hammer is released by its sear, and accelerates forward to hit the rear end of a firing pin which passes through the breech face of the gun and makes contact with the primer. The firing pin hits the primer hard enough to dent the metal, and this causes the unstable chemical inside to ignite. A jet of flame from the primer rushes forward to ignite the nitro-cellulose propellant, which burns rapidly to form a large volume of hot, expanding gas.

This high gas pressure forces the wad and the shot through the barrel, usually accelerating it to supersonic speed, and you hear the bang as the shot, wad and hot gas exit into the open air.

And you maybe thought the gun went 'bang' simultaneously with your pulling the trigger? It does feel like that, but the whole process can be timed, and it takes a few milliseconds - far too short a time to be appreciated by human senses.