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Guideline: Understanding Gunshot Residue and Propellant

Introduction

Some of the most brutal crime scenes I’ve experienced during my days as a forensic expert in the South African Police Services (Saps) were where a firearm was used.  This was either in the form of a victim or victims who were shot and severely injured, or the victim or victims were fatally wounded. Then of course there are those who committed suicide. In all the above as mentioned, the firing of a firearm constituted the release of gunshot residue (primer residue) and propellant.

To the uninformed or untrained individual, the importance of such residue and propellant are often missed, even at times to members of the Saps or security personnel who arrive first at such scenes. Crime scenes must be well managed in order to persevere with any form of evidence. Many times, cases are thrown out of court either due to a lack of evidence or whereby evidence was actually contaminated due to various ignorant and sometimes purposeful acts at the crime scene. My guideline is addressing the importance of primer residue and propellant in order for all those coming across such a crime scene to understand the importance of it as evidence and in the process is able to preserve the crime scene.

When the trigger of a firearm which is opening normally, is depressed, the mechanism causes a hammer or firing pin to strike the base of the cartridge in the breech, resulting in a depression of that part of the cartridge case. In the case of a rimfire cartridge, a section of the rim is struck, whilst in a centerfire cartridge, the central area is struck. In both cases, the energy of this mechanical action initiates a series of chemical reactions, resulting in the projectile being propelled from the cartridge case.

The case of a rimfire (.22 calibre) cartridge has a groove around the inside of the rim. A shock-sensitive chemical mixture known as the primer mixture, is applied to this groove. The propellant is then placed into the cartridge case and the bullet is clamped into the top. The bulk volume of the centerfire cartridge cases is taken up by the propellant chamber. As with rimfire cartridges, the propellant chamber is sealed with the bullet. The propellant and primer chambers are connected by one (Boxer), two or three (Burdan) holes known as flash holes.   

When the hammer or firing pin strikes the rim of a rimfire cartridge, the inside of the rim is compressed, whereas in a centerfire cartridge, the base of the primer cap is forced towards the anvil. The resulting compression of the primer mixture initiates the shock sensitive reaction. The energy of this reaction, in turn, initiates the rapid combustion of the propellant. These reactions produce very high temperatures between 1 500 and 2 000°C and a pressure +- 10 4kpa that causes the components of the primer mixture to vaporize. 

As the vapors expand and cool, the reaction products condense to form small particles. The latter is ejected predominantly in a forward direction along the barrel, but also backwards and sideways. In this way, primer residue is expelled from the muzzle, breech and any other ports or clefts in the firearm. The process results in the disposition of primer residue on the hands and other parts of the shooter as well as on the target should the target be nearby. A variety of techniques are used for the detection of firearm gasses. The techniques usually employed were designed for either primer residue or propellant residue tests. 

Table of Contents

  1. Gunshot residue (GSR)-Primer Residue (PR)

  • Composition and Morphology

The morphology (shape) of the particles together with the chemical composition makes these particles unique. The composition consists primarily of silicon, antimony, barium and lead. Unique implies that these particles can be positively identified as being only from the primer of a cartridge. Primer residue is spheroidal and very small, with an average size of 1 to 10 micrometer. These particles could easily get lost, due to hand activity, since they are only laying loose on the hands. 

  • Hand Grenades

Hand grenades also have primer residue. The mechanism works as follows:

The safety pin is pulled out, and once the grenade is lobbed, the safety lever is released. The firing pin spring is allowed to expand and to fire the primer. Primer residue is released, the timer fuse burns and the grenade will explode after the set time which is around 10 seconds.

The amount of primer residue on the hand of a person throwing a hand-grenade will be less than that of a person firing a firearm, due to the fact that the grenade is armed only when it leaves the hand of the thrower. The chemical composition of the primers in hand grenades are the same as the chemical composition of sinoxide primers   

  • What does it mean when a person tests positive for primer residue

When a person tests positive for primer residue, it could be as a result of one of the following reasons:

  1. he fired a firearm,
  2. he was within a radius of 1,5m to 2m from a person who fired a firearm, 
  3. he handled a recently fired firearm.

When a person’s left hand tests positive for primer residue, it doesn’t necessarily indicate that he is left handed or that he used his left hand to fire the firearm. 

Photo: Primer GSR on a smokeless powder disk (white spots)
  • What does time lapse mean

Time lapse is the time between the discharge of a firearm and the time the primer residue test was performed. The maximum time lapse is 2hrs 30 min. In the case of a deceased, there is no time lapse.

  • Performance of the primer residue test in the case of a living suspect

  1. The forensic specialist must be at the crime scene as soon as possible, within 2hrs 30 min.
  2. The forensic specialist must, before he/she starts sampling, wash his/her hands and every time between suspects in order to prevent contamination.
  3. The suspect must not be allowed to wash his/her hands
  4. Do not handcuff him/her before the test is done
  5. Do not allow freedom of movement of the hands (e.g. putting in pockets, taking off a jersey etcetera.
  6. Do not allow him/her to touch the hands of the deceased
  7. Do not allow him/her to handle the suspected weapon or any other weapon
  8. In the event of a chase, prior to the suspect’s arrest, there must also be a primer residue test performed on the clothing
  9. If a pistol or revolver was used, do the primer residue test on the sleeves of a long sleeve shirt or jacket: or on the forearms in the case of a short sleeve 
  10. If a long barrel gun was used, do the primer residue test on the collar of the face, the cheek area
  • Performance of the primer residue test in the instance of a deceased

  1. Do not move the body i.e. do not move to the mortuary before the samples are taken
  2. Do not touch the hands of the deceased
  3. Do not step over the body
  4. Take photos of the hand positions
  5. Where the hand is covered with blood, do the test also on the forearms   
  • Performing the primer residue test when shots were fired from a vehicle

  1. The test must be conducted on the inside of the roof
  2. It must also be done on the window sills, sun visors, headrests and dashboards
  • Primer residue samples send in together with firearms and cartridges, will  not be analyzed

1.8.1 In all shooting incidents, samples must be taken from both hands of all suspects in order to establish the presence or absence of primer residue particles

1.8.2 The test should be performed on the hand between the thumb and the index finger

1.8.3 The best chance for a positive result is when samples are taken as soon as possible within 2hrs and 30min after the incident 

Case Law

S v Mopedinyane and others (SS147_2017) [2018] ZAGPJHC 614

(3 September 2018)

DISCUSSION ON GUNSHOT RESIDUE (GSR)

[87] “I have deliberately extended the judgment in dealing at length with the issue of Gunshot Residue (GSR), and on which much has not been researched and written, purely in order to ensure that the correct verdict was pronounced in the present matter.  Indeed, there are other foreign and international research articles, some of which are critical, and sceptic about evidence of GSR, and recommends a cautious approach thereto, which are all rather instructive.  For example, in Forensic Sciences, Critical Review Criminalities (available on line at: onlinelibrary@wiley.com), at page 13, the learned authors opined that, “the presence of GSR on an individual is not exclusively indicative of firearm discharge … Investigations have high-lighted the possibility for GSR to undergo secondary, and even tertiary, transfer (62) and also the potential for GSR particles to deposit onto an individual walking through a recently contaminated area”. 

[88] Back to the present matter. The facts in the above case law are obviously distinguishable from the facts in the present matter. The taking of the samples from the accused, the storage, the transportation and analysis thereof were not seriously challenged.  The only lame challenge emanating from accused 1 and 4, was that at the time of the taking of the samples, the accused still had blood on their hands as a result of the dog bites, and that this could have led to the compromise and possible contamination of the samples.  No authority was proffered to the Court for the proposition; neither could I find any in my extended research. Other than the above, the accused implicated merely denied the results of the GSR analysis, and failed dismally to explain why it was found present on their hands. I must accept the results of the GSR analysis in these circumstances.  There was no counter expert testimony presented.  I must also at this stage, make the specific finding that, the murder allegations, where the accused were proved to be involved, showed that the murders were not only pre-planned, and pre-meditated, but also carried out with precision and in pursuit of a common purpose, all seemingly based on the rivalry and competing interests and rivalry at the Genesis mine dumping site.”

Judge DSS Moshidi

  1. Propellant

The ammunition of modern rifles and weapons consist of four basic components:

  • the copper case
  • the primer
  • the propellant and 
  • the bullet

The propellant particles are distributed in a cone shaped form from the barrel. The modern propellant burns fast and is smokeless, it also replaces the old black gunpowder. The design of modern propellants contributes to the total combustion of the propellants, of which the products are nitrates.

The amount or to what extent the propellant particles will be on target, will in both instances depend on the target. Propellant will easily remain on clothing, but not necessarily on a piece of glass. The particles will, depending on the rigidity of the target, either be shot into or fall off the target.

2.1 Dispersion pattern of propellant

2.1.1 The dispersion of the propellant, if it is close enough to the target, is a function

of the weapon and ammunition

2.1.2 Most of the soot, smoke and propellant particles are under high pressure, which leaves the barrel in a volume

2.1.3 The volume enlarges, which could cause the target to tear

2.1.4 A specific weapon will disperse the gasses, smoke, root and unburnt propellant particles in a fairly constant fashion in a cone shaped form from the barrel

2.1.5 The substances will be carried onto the target over a short distance

2.1.6 The strength of the target will greatly determine the appearance of the hole in the target.

2.1.7 The presence of smoke and soot on the inside or back of the target gives an indication that the target was either penetrated or perforated

2.2 Distance determination

2.2.1 The distance of the weapon from the target could be determined by the presence, appearance and distribution of the substances, by doing tests with the specific weapon and ammunition.

2.2.2 A shot fired near the target will have a prominent smoke and soot deposit

2.2.3 The moment a shot is fired with a specific weapon, the gasses that come into contact with the atmosphere, will produce a flame

2.2.4 If this flame, of which the size is dependent on the weapon used, is close enough to the target, the target will be blackened and partially burnt

2.3 Examination methods for distance determining

2.3.1 The clothing of a deceased is the most important exhibit used for the determination of distance

2.3.2 The clothing is examined optically such that the dispersion pattern of the propellant, as well as the density and diameter thereof could be determined.

2.3.3  In spite of the fact that clothing are often stained with blood, the dispersion pattern of the propellant could be determined by means of infra-red image analyzer 

2.4 Chemical test for propellant

2.4.1 Chemical tests confirm that which was observed on the target, was indeed propellant.

2.4.2 The partly burnt propellant particles contain nitrite, and these tests specifically test for the presence of nitrites

2.4.3 Propellant is manufactured by the nitration of cellulose with sulphuric and nitric acid

2.4.4 These nitrates form the nitrites during combustion

2.4.5 Nitrites are not unique to propellant, it can also be found in manure

2.4.6 The dispersed propellant particles and concentration should be interpreted with other chemical information.

2.5 Collection of exhibits

Contamination is a big problem with the determination of propellants. Causes for contamination are the handling of exhibits without gloves.

2.5.1 If the entrance wound was through clothing, the clothing should be placed in clean exhibit plastic bags

2.5.2 If the entrance wound was through the skin, the following procedure should be followed:

2.5.2.1 take well illuminated photographs close to the wound area

2.5.2.1 take well illuminated photographs of the general wound area (30cm x 

30cm. A lot of conclusions could be made from the form and tattooing

 as a result of burns  

2.5.3 Use clean cotton wool, acetone, and gloves for the acetone smear around the wound.

2.5.4 The wound should be smeared thoroughly 

2.5.5 The cotton wool should be put into a container which is acetone resistant

 

Conway Evertson

Conway Evertson is a former Director of Community Safety, a Senior Lecturer (Criminal Justice), Forensic Expert (SAPS), Head Investigations (IPID) and an Infantry Instructor (SADF).

 

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