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This essay might be of special interest to writers of detective and mystery novels who would like to enrich their stories by providing their readers with a gift of extra details. It might also be of general interest to many other readers, especially those who are CSI and NCIS fans.


Lou was in his garage. He had just finished replacing worn brake pads on his four-year-old, black Ford sedan and was wiping his tools with a rag when two Los Angeles Police detectives came for him. He was held at gunpoint while being fitted with a shiny pair of bracelets connected by a short, but fashionable, chain. He was searched for weapons and was told that he was suspected of having shot and killed a shop keeper during a robbery earlier that day. He was read his Miranda rights and treated to free transportation downtown.

Interrogators downtown told him that two passersby had seen a man fitting his description and wearing jeans and a dark shirt hurry out of the shop and drive away in a black Ford sedan. One had even been able to provide the last three numbers on the car’s license plate. Lou was wearing jeans and a dark shirt, and the description of the person leaving the shop fit him. The description of the car and the three numbers on its license plate had enabled the detectives to trace Lou’s address.

A .32 caliber bullet was found to have killed the shopkeeper. Under subsequent interrogation, Lou admitted that he owned a .32 caliber, semiautomatic pistol, but he claimed it had been lost during a recent move. He said he didn’t think it had been stolen, just misplaced during the move. He figured it would turn up eventually when remaining moving boxes were unpacked. He had not been concerned about its location because he had no interest in firing it or even keeping it any longer. A later search of his house and garage failed to find it.


A cartridge comprises a casing within which is held an explosive powder. A bullet is held in the forward end of the casing, and a primer is disposed at either the center or rim of the base of the casing. Typically, a bullet is formed of lead, which is sometimes jacketed with metal. The primer is struck by a firing pin driven by a gun’s released hammer. The primer contains a charge of impact-sensitive material that explodes and ignites the explosive powder when the gun is fired. Hot, expanding gasses generated by the exploding powder propel the bullet along the barrel of a gun and send it on its way.

A bullet is not alone as it leaves the barrel. It is accompanied for some distance by possible combinations of material from the primer, the exploded powder, lubricants and various metals from the bullet, its jacket, the cartridge and the gun barrel. These combinations form what are collectively known as gunshot residue (GSR). Most primers contribute residue deposits containing lead, antimony and barium. A bit of unburned explosive powder also usually joins the parade of gunshot residue through the barrel.

Gunshot residue consists of very small particles that have little mass. Once free of the barrel, they lose momentum quickly and stray from the path of the bullet. As a result, when a person fires a pistol, gunshot residue can usually be found on the back of the person’s hand, on his or her face and clothing and even in the person’s hair.


Lou was found to have traces of lead, antimony and barium on his hands and clothes. During a subsequent trial, he was charged with first-degree murder. In most states, a first-degree murder charge requires a killing to be unlawful, willful and premeditated. The killing was not held to have been premeditated in this case, however, because the shopkeeper’s body was found with a gun in it’s hand.

In California, as is the case in many other states, there exists a felony murder rule. In California, under this rule, a person is considered to have committed a first-degree murder if a death results from the commission of certain violent felonies. According to Section 189 of the California Penal Code, any “murder perpetrated by means of a destructive device or explosive, a weapon of mass destruction, knowing use of ammunition designed primarily to penetrate metal or armor, poison, lying in wait, torture, or by any other kind of willful, deliberate, and premeditated killing, or which is committed in the perpetration of, or attempt to perpetrate, arson, rape, carjacking, robbery, burglary, mayhem, kidnapping, train wrecking, or any act punishable under the sex offense sections of 206, 286, 288, 288a, or 289, or any murder which is perpetrated by means of discharging a firearm from a motor vehicle, intentionally at another person outside of the vehicle with the intent to inflict death, is murder of the first degree.”

Fortunately for Lou, studies had discovered that environmental particles and some types of brake linings and their wear products also contain lead, barium and antimony and present gunshot-residue-like elemental profiles. The residue from brake sources can usually be differentiated by the presence of high levels of iron and/or the presence of prohibited materials, but some brake-pad wear products have been found that lack this combination. This discovery can cast doubt on the source of gunshot-residue-like particles found on a person. Such doubt was instrumental in a jury finding Lou not guilty of the shop keeper’s murder.

Some time later, another person who looked similar to Lou and also drove a black sedan having the same three last license-plate numbers, was arrested for and confessed to having shot the shop keeper. Lou’s car was a black Ford. The killer’s car was a black Chevrolet.


Second degree murder is generally defined as an intentional killing that is not premeditated or committed in a reasonable heat of passion. It can also be a killing resulting from a person’s dangerous conduct and obvious lack of concern for human life. Second-degree murder holds a position between first-degree murder and voluntary manslaughter. An example would be when a person fires a gun at one person and accidentally hits another.

Voluntary manslaughter is generally defined as an unplanned, spur-of-the-moment killing of someone a person had not previously intended to kill. It must occur as a result of circumstances that would cause a reasonable person to become emotionally out of control.

An interesting, and possibly hypothetical, example of the scope of the felony murder rule might include two felons who committed an armed robbery. As they were fleeing the police, the driver of their car lost control and crashed the car into a tree. The felon driver was killed. The felon passenger survived and was charged with the first degree murder of the driver for having taken part in what was the ‘proximate cause’ (the robbery) of the driver’s ultimate death. An important reason for adopting such a rule was to discourage the commission of gang-perpetrated felonies.

In addition to determining whether or not a person has fired a gun, gunshot residue is also used to locate bullet holes and to estimate firing distances.

Gunshot residue materials are usually identified using one of two methods. Bulk analysis uses atomic absorption spectroscopy or neutron activation. Resident particle analysis uses scanning electron microscopy and energy dispersive X-ray analysis. (This information will not be included in Friday’s exam.)

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