Lesson 3 - Testing for Gunshot Residue

Testing for Gunshot Residue

Testing for Gunshot Residue

The Main Parts of a Firearm Cartridge

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Testing for Gunshot Residue

When the trigger of a gun is pressed, the firing pin strikes the primer (5) at the base (4) of the cartridge. This causes a combustion reaction that produces flames. These ignite the gunpowder or propellant (3). The burning gunpowder produces a large amount of gas that propels the bullet (1) from the gun barrel, leaving the casing (2) behind.

Gunshot residue is composed of two substances: a propellant and a primer. Each of these is a solid mixture made of various chemical compounds. The two types of propellants are black powder and smokeless powder.

Black powder consists of charcoal (15%), sulphur (10%), and potassium nitrate (75%). When black powder is ignited, about 55% of its products are solids (soot). The remaining 45% is a gaseous thick black smoke. The soot left behind corrodes the interior of the gun, and the thick smoke reduces accuracy because the target may be obscured.

Black gunpowder is used to blast granite and marble from the earth because it causes fewer fractures than other explosives.

Smokeless powder is the name given to a number of gunpowder-like propellants that do not produce smoke when fired because their product is mainly gas. All modern ammunition uses smokeless powder. Various types of smokeless powder contain either nitrocellulose (single-based powder) or nitrocellulose mixed with nitro-glycerine (double-based powder).

The granules that make up smokeless gunpowder are coated with graphite to prevent static electricity from causing undesired ignitions.

A primer is a mixture of chemical compounds located in the base of a cartridge. It ignites when subjected to great pressure such as when struck by the firearm’s firing pin.

Several types of primers are available, but the most common are lead(II) azide, lead(II) styphnate, mercury(II) fulminate, barium nitrate, potassium chlorate, and antimony(III) sulphide.

Originally, bullets were spherical metallic or stone balls placed in front of a charge of black gunpowder at the end of a closed tube.

Glossary Term: Combustion

  • A chemical change accompanied by the production of heat and light

Glossary Term: Propellant

  • Something that propels or provides thrust; especially gunpowder

Glossary Term: Black Powder

  • Consists of charcoal (15%), sulphur (10%), and potassium nitrate (75%). When black powder is ignited, about 55% of its products are solids (soot). The remaining 45% is a gaseous thick black smoke.

Glossary Term: Smokeless Gunpowder

  • The name given to gunpowder-like propellants that do not produce smoke when fired because their product is mainly gas

Glossary Term: Primer

  • Located at the base of the cartridge. Impacting the primer with the firing pin causes a combustion reaction that produces flames, and in turn discharges the round

Glossary Term: Firing Pin

  • The part of the bolt or breech of a firearm that strikes the primer and detonates the charge of a projectile (bullet)