Gunshot Residue

When the firing pin of a firearm strikes the primer of a cartridge the primer compound ignites sending a flame into the cartridge case.  Gunpowder in the cartridge case starts to burn, causing it to change from a solid material to a gas.  This change creates pressure within the cartridge, which in turn forces the bullet down the barrel and down range. Pressure building behind the bullet is released when the bullet exits the muzzle of the firearm.

The bullet acts like the cork in a shook up Champagne bottle.  When the bullet exits the muzzle, pressure behind it blows the gunshot residues out of the firearm's barrel under high velocity. The residues are expelled from the barrel in a smoky cone shaped pattern.

Time-lapsed image showing a bullet exiting from the barrel.  Streaks of burning gunpowder, smoke, and unburned particulate can be seen exiting the barrel as well. 

The further gunshot residues travel from the muzzle, the broader and less concentrated the pattern becomes.  Because the various elements included in gunshot residues are very small and lack mass they lose their energy rapidly.

Gunshot residues can also be emitted from other areas of a firearm. As you can see in the above image, gunshot residue is escaping from the barrel/cylinder gap of the revolver.

The muzzle-to-garment distance can vary considerably depending on the firearm and type of ammunition being used.  Short-barreled firearms and lower velocity cartridges will not normally expel residues as far as a high velocity rifle.  At shorter distances however, they may deposit greater concentrations of gunshot residues.  Also, gunpowder can come in several forms such as ball, flake, disc, and others.  Ball powder being spherical in shape is more aerodynamic than say a particle of flake gunpowder and as a result will travel farther.  A number of other variables can influence the amount of gunshot residues that may reach a target; therefore, it is essential that the firearm and ammunition used in the shooting incident be recovered.

Gunshot residues emitted from the muzzle will travel out to distances of approximately 3 and 5 feet in most firearms but in some cases can travel even greater distances.  At the 3-5 foot range the gunshot residues may only consist of a few trace particles and make determining the firing distance difficult if not impossible.

As the firearm gets closer to its target the residue concentrations increase and the actual size or diameter to the pattern gets smaller.  At around 18-24 inches most firearms will start to deposit considerable concentrations of gunshot residues that may or may not be visible to the eye.

At distances of less than around 12 inches heavy concentrations of visible gunshot residues will normally be deposited.

Visible gunshot residues around bullet entrance hole.

When the muzzle of the firearm gets next to or is in contact with the target, hot gases escaping from the muzzle at high velocity will typically rip, tear, shred, and/or melt the material of the target.  A very intense deposit of gunshot residues will be found around the margins of a contact or near contact entrance hole.

Contact gunshot entrance hole.

There have actually been cases where a hard contact gunshot (muzzle pressed hard against the victim) caused the residues to blow through the wound tract in the victim and be deposited around the inside of the exit hole of the victim's clothing.

Gunshot residue is normally a combination of gunpowder residues and lead residues.  I say normally because some newer ammunition is virtually lead-free.  More and more ammunition manufacturers are using lead free or low lead propellants because of the toxicity of lead.

Gunpowder residue can contain unburned gunpowder particles, partially burned gunpowder particles or the carbonaceous soot from completely burned gunpowder.  The image below show a bullet hole surrounded by gunpowder particulate residue.

Gunpowder particulate residue around bullet entrance hole.

Modern smokeless gunpowder, and black powder, contains nitrate compounds.  Black powder normally contains a combination of potassium nitrate (75%), charcoal (15%), and sulfur (10%).  Smokeless powders can either be single based or double based.  Single based gunpowder will contain nitrocellulose (cellulose hexanitrate) as its main ingredient.  Double based gunpowder contains nitrocellulose and nitroglycerin (glyceral trinitrate) as its base.  Some triple-based powders are also now available.

When either of these types of gunpowder burns the residue left behind will be in the form of a nitrite-based compound.  Nitrite particles when emitted from the muzzle of a firearm will strike a nearby target and either be imbedded in the target's surface or leave a deposit of nitrite residue.

Lead residues will be in a vaporous or particulate form and can come from a couple sources within a discharged cartridge.  The most common source is the primer.  Primers are used to start the ignition process in cartridges and commonly contain lead styphnate, barium nitrate, and antimony sulfide compounds.   However, some newer primer compounds are being used that are lead and/or barium free.

Cartridges containing lead based primers, when ignited, produce a vaporous cloud of residue that is expelled from the muzzle of the firearm.  Additional vaporous lead residues can be produced when the hot gases pushing a lead bullet down a barrel melt lead from the base of the bullet.

A third form of lead residue will be in a particulate form.  Particulate lead residue comes from minute lead particles that are shaved from the sides of a lead bullet as it travels down the barrel.  Lead particulate has more mass than vaporous lead and travels greater distances.  Also, gunpowder particles can be coated by the vaporous lead residues and leave what appears to be a lead particulate deposit upon striking the target.

The amount of lead residue emitted from a gun can vary slightly from shot to shot.  Fouling in the barrel from previous shots can slightly increase the amount of lead residue emitted from one shot to the next.

As described above, gunshot residue can be deposited on articles of clothing when in close proximity to a discharged firearm.  But will it stay there?  In most cases the answer is yes.

The various elements contained in gunshot residue are not readily water soluble and clothing left exposed to the elements will not usually diminish the residue deposits.  Other factors such as heavy bleeding and rough handling of the garment can cover up or dislodge some residues.  This has to be taken into consideration when conducting all such examinations.  The garments must be promptly collected, allowed to air dry, and packaged in a way that will minimize contamination.

The clothing submitted to the laboratory will be examined to determine if a pattern of gunshot residue is present and there are a number of examinations conducted to aid in this determination.  Click next below to learn more.


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