Our most recent Newsletter “Gunshot Residue Analysis” provides a detailed analysis of the topic while the following blog article provides a cursory review of the topic. You can also sign up here to receive our Forensic Science Newsletter by email.
Gunshot residue – often abbrevieated as GSR – is the material that is deposited on the hands or clothing of someone who has:
- Discharged a firearm.
- Was near a firearm as it was discharged.
- Handled a recently discharged firearm.
- Touched an object that was handled by a person who has discharged a firearm.
In addition, GSR may be present on an entrance or exit wound, or on materials that were targeted by a firearm.
GSR is composed of particles from a gun’s cartridge primer, as well as gun powder, minute fragments from the cartridge case, the surface of the bullet and lubricants used on the gun.
Remember, GSR may not always be visible. Particles could be microscopic. The presence of black material on a shooter’s hands or clothing may not always indicate the presence of GSR, just as clean hands or clothes does not indicate the absence of GSR.
The search for GSR requires painstaking work, whether collecting evidence at the scene, during an autopsy and during technical analysis of the evidence. It needs to be done by qualified professionals with the appropriate equipment.
What happens when someone fires a gun?
In our discussion of GSR, It’s important to understand what happens when someone fires a weapon.
When you pull the trigger on a firearm, the firing pin strikes the primer cap, causing an explosion and flame.
The flame resulting from the explosion passes through holes in the primer’s anvil, igniting the propellant in the cartridge case. The ignition causes a high temperature and pressure reaction, propelling the bullet from the barrel of the gun.
Small arms cartridges are made up of a primer, gunpowder (propellant), the cartridge case, and the bullet. Let’s take a closer look at each component.
Primer case and primer
The primer case is normally made of zinc and copper, which can both be detected. Small arms cartridges come in two categories, depending on the location of the primer: centerfire and rimfire.
Centerfire primer compound is made of three major chemical components, which play a major role in forming and identifying GSR:
- Lead styphnate, which is the initiator, is set off when the firing pin hits the primer cap.
- Barium nitrate, the oxidizer, which gives off the oxygen needed to burn the fuel.
- Antimony sulfide is that fuel, which burns at a high rate, igniting the gun powder, which propels the bullet.
With rimfire cartridges, the composition of the primer varies with the manufacture. Most U.S. makers of .22 ammunition use lead and barium in their primer. An exception to this rule is the Remington company, which uses a primer made of lead, barium and antimony. The trend now is to go toward lead free primers.
Propellent and cartridge case
The cartridge case, like the primer case, is typically made of copper and zinc, although some cases have a nickel coating.
The gun powder used in modern propellants contains as many as 23 organic compound, which can be identified. For example, reagents containing sulfuric acid can detect diphenylamine, one of the stabilizers used in gun powder.
Today’s gun powder – known as smokeless powder – was first developed in the late 1800s. One variety was invented by the French chemist, Paul Vielle in 1884, another by Alfred Nobel three years later. Nobel’s version used nitrocellulose, which is not as highly nitrated as the powder developed by Vielle.
Winchester’s ball powder, introduced in 1933, also uses nitrocellulose, however, it is dissolved completely instead of being colloided. It also burns differently than flake powder.
Bullets fall into two categories: metal-jacketed and lead-jacketed.
- Lead bullets are, as the name suggest, made from lead. Antimony may be added to increase their hardness, and some have a ferrous alloy core. They may be coated with nickel, copper, or a copper alloy. They are lubricated with grease, and have a hard metal cup known as a gas check.
- Metal-jacketed bullets are usually copper or brass, with a lead or steel core. Some have covers made from metal clad steel, copper-nickel or aluminum. Frangilble bullets are usually made of copper and a synthetic polymer.
All of these components can be detected through the proper analysis. Among the methods are flameless atomic absorption spectrometry, inductively coupled plasma-mass spectroscopy, which are known as bulk analytic techniques, good for detecting trace amounts of elements such as lead, barium and antimony at the nanogram range (that’s one billionth of a gram).
There are also particle-by-particle analysis like scanning electron microscopy, which can detect a single, micron-ranged GSR particle.
To learn more about these methods and GSR, be sure to read the latest installment of our Forensic Science Newsletter -“Gunshot Residue Analysis.” You can also sign up to automatically receive our Forensic Science Newsletters.