Firearms: The Science

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1.0 Introduction

1.1 Legal Definition of Firearm

  • For the purpose of the Firearms Act and for offences related to the Firearms Act in the Criminal Code, a "firearm" is:
    • A barreled weapon from which any shot, bullet or other projectile can be discharged and that is capable of causing serious bodily harm injury or death to a person [1]: 
      • This definition includes any frame or receiver of such a barreled weapon, as well as;
      • Anything that can be adapted for use as a firearm
  • However, there are some weapons and devices that meet the definition of a firearm but are not deemed to be firearms for the purpose of the Firearms Act and related offences.  Some of these exemptions only stand if the items were designed exclusively for a specific purpose and are intended to be used exclusively for that purpose.  However, all of the following items are considered to be firearms if they are used in a criminal or negligent manner [2]:                                                                                                                      
    • antique firearms, defined as:
      • any firearm manufactured before 1898 that was not designed to discharge rim-fire or centre-fire ammunition and that has not been redesigned to discharge such ammunition, or;
      • any firearm that is prescribed to be an antique firearm [3]
      • devices designed exclusively for signaling, for notifying of distress, for firing blank or stud cartridges, explosive-driven rivets or other industrial projectiles
      • shooting devices designed exclusively for slaughtering domestic animals, tranquilizing animals, or discharging projectiles with lines attached to them
      • air guns and other barreled weapons designed to have:

    1.2 Basic Terminology and Classification

    Generally speaking, firearms can be divided into two types: handguns and shoulder firearms. The handgun class encompasses revolvers and pistols. Shoulder firearms are a more diverse class, and encompass rifles, machine guns, submachine guns and shotguns.

    Although there are a great variety of firearms, with dramatically different designs and ultimate purposes, the basic anatomy of each and every firearm is designed based on one common goal—the firing of ammunition.

    Speaking generally once again, ammunition is typically composed of the following: one or more projectiles, propellant (to act as fuel), and a primer (to ignite the propellant). As with firearms, there are two major types of ammunition: cartridges, for handguns and rifles; and shells or shotshells, for shotguns [4]. 

    Note: the word “cartridge” refers to the cylindrical casing containing the primer and propellant for a firearm. Cartridge can be and sometimes is used to describe the ammunition casing for both handguns/rifles AND shotguns. However, the term “shotshell” or “shell” is more commonly used to describe the “cartridge” of shotgun ammunition.

    2.0 Ammunition

    2.1 Projectiles

    Bullets: Bullets are one type of projectile, and can be classified generally as lead, lead-alloy, semi-jacketed or fully-jacketed, and further as center-fire or rim-fire.

    Lead bullets are soft and may undergo extreme deformation or fragmentation upon hitting a target. They are only used in low-velocity firearms (the significance of velocity in firearms will be discussed later). Lead-alloy bullets contain elements like antimony or tin to make them harder and can thus be used in higher-velocity firearms, but they too readily deform on impact [5].

    Semi-jacketed bullets usually have a lead core covered with a thin jacket of brass. The brass jacket typically covers the side of the bullet but leaves the lead core exposed at the nose or tip (although there are some bullets that cover the nose but expose the base) [6]. Because the tip of the bullet is softer than the jacket that surrounds it, the tip expands (like a mushroom) on impact. This expansion effect is increased in “hollow-points bullets”, where the tip of the bullet is hollow [7].  These bullets are deliberately fragile—the expansion causes the tip to expand even more or even shatter, thus increasing the extent of damage. Hollow point bullets were designed for hunting big game, and there were restrictions on their import, but since the restrictions were lifted they have become one of the most commonly used types of ammunition [8]. 

    Fully-jacketed bullets have their lead core entirely covered by a harder metal jacket, which significantly reduces the expansion of the bullet both upon entering the barrel and hitting the target [9]. 

    Firearms examiners may also encounter a number of unusual or special purpose bullets, for example frangible or pre-fragmented bullets which are composed of particles or pellets of metal embedded in a binding agent such as plastic. The binding agent in pre-fragmented bullets is designed to disintegrate on impact allowing the pellets to disperse for greater damage [10]. 

    Centre-fire vs. rim-fire refers to where the bullet must be struck by the firearm’s firing pin in order to ignite the primer.

    The jacket and cartridge casing of a bullet become very important in forensic firearm examination, as can its weight, size, composition and other factors.

    firearms1.gif

    Image courtesy of www.firearmsID.com, artwork by Erik Dahlberg.

    Shot: Shotguns can fire a variety of projectiles, but the most common is shot. The term shot refers to small balls of metal, usually lead, lead alloy or steel. The size of the pellets used may depend on the size of the target, or the firearm. The shot is loaded into a casing called a shell or shotshell. Modern shell casings are plastic with a metal head. A plastic or fiber disc called a wad separates the shot in the shell from the propellant. Some shells have a second wad placed over the shot, and the mouth of the shell (the end) is “crimped” together in order to seal it. Shells may also contain a plastic cup that keeps the shot together and protects it from getting deformed via contact with the shotgun barrel. Wads and cups may become secondary projectiles—sometimes, if the shot was at close range, they can be recovered from the resulting wound itself. They can also inflict superficial injuries near the wound [11]. 

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    Image courtesy of www.firearmsID.com, artwork by Erik Dahlberg.

    A shotgun can also fire a slug, which is usually a single heavy sphere-shaped lead projectile, similar to a bullet. The anatomy of the shell remains the same, but the shot is replaced with the slug. Shotgun cartridges may also contain pellets made of plastic or rubber, or tiny arrows called flechettes.

    NB: Older black powder rifles and shotguns are muzzle loading—the components of the modern shotshell are separate and inserted manually through the muzzle of the firearm using a series of rods. 

    2.2 Propellant

    The propellant is the fuel that ignites to propel the projectile down and out of the firearm’s barrel. The first main propellant was black powder (most commonly made up of charcoal, potassium nitrate and sulfur), which was invented centuries ago. Although it has been largely replaced by newer formulations, it remains in use with some black powder hunting or antique weapons enthusiasts [12]. 

    The more common by far propellant is smokeless powder, which does not produce the large plumes of smoke common with black powder. It is also chemically stabilized for safe manufacture and transport [13]. 

    2.3 Primer

    The primer is what ignites the propellant. It usually consists of a small metal cup or capsule which contains a percussion-sensitive material (i.e. something that explodes on impact) that, when struck, explodes and create enough heat to ignite the propellant. The cup containing the primer is set at the base of the cartridge, where it can be ignited by the impact of the firing pin [14]. 

    3.0 Firearms

    3.1 Handguns

    3.1.1 Revolvers

    In a revolver, the cartridges are held in separate firing chambers within a rotating cylinder.

    Single-action revolvers are fired by manually cocking back the hammer, which both rotates the cylinder to place one of the chambers in front of the firing pin (the firing pin is on the tip of the hammer) and cocks the firing mechanism (as it moves backward, the hammer compresses a tension spring). When the trigger is pulled it releases the hammer and the spring, and the force of the release drives the firing pin into the base of the cartridge. The impact of the pin ignites the primer, which in turn ignites the propellant. As the accelerant burns it releases gas, the pressure of which drives the projectile (the bullet) down the barrel. The cylinder is rotated to expose the next cartridge by a pawl and ratchet mechanism attached to the trigger. When all the cartridges have been fired, the cylinder is swung out and the empty casings are removed and replaced [15]. 

    Double-action revolvers work in nearly the same way, except the hammer is not cocked manually—instead, one long trigger pull raises the hammer, moves a firing chamber underneath it/in line with the firing pin and then allows the hammer to drop, releasing the spring mechanism and firing the cartridge [16]. 

    The interior surface of the barrel of a revolver is rifled with a series of ridges and grooves that spiral down the length of the barrel—these ridges are technically referred to as lands and grooves. The bullet is forced into a spin, which stabilizes its flight. Rifling is very important to forensic firearm identification as the grooves are distinct and leave marks on the bullet, called striations, which can be used to match a bullet as shot from a certain gun.

    firearms3.JPG

    A Colt revolver.

    3.1.2 Semi-automatic Pistols

    These firearms are sometimes called autoloaders or self-loaders, due to the fact that, once one shot has been fired, the mechanism of the gun works to automatically leave it both cocked and loaded—pulling the trigger immediately fires the next shot. They are called semi-automatic because a true automatic pistol would continue to fire as long as trigger was depressed and the magazine contained ammunition [17].

    One of the most obvious differences between a semi-automatic pistol and a revolver, for the layman, is that semi-automatic pistols eject the spent cartridges, whereas with a revolver they remain in the cylinder and must be removed manually.

    In a semi-automatic pistol, the cartridges are held in a removable magazine contained within the grip, and are loaded sequentially into the pistol from the top of the magazine. In a typical firing sequence, a cartridge is chambered into the pistol, usually by manually moving the slide to the rear, and then allowing it to move forward, catching the top cartridge from the magazine. This simultaneously cocks the firing mechanism (which is very similar to that of a revolver—the hammer is pulled back, compressing a spring which, when released, drives the firing pin into the newly-chambered cartridge), and when the shooter pulls the trigger the pistol fires. A portion of the explosion energy of the propellant automatically forces the slide back to the rear, which extracts and ejects the spent cartridge, and causes another spring mechanism to push an unfired cartridge from the magazine up into the firing chamber [18].

    The interior surface of the barrel of a semi-automatic pistol is also rifled with a series of lands and grooves (ridges) that spiral down the length of the barrel. The bullet is forced into a spin, which stabilizes its flight. Rifling is very important to forensic firearm identification as the grooves are distinct and leave marks on the bullet, called striations, which can be used to match a bullet as shot from a certain gun.

    Image courtesy of www.firearmsID.com, artwork by Erik Dahlberg.

    3.2 Rifles

    Rifles work more or less the same way as handguns, or any other firearm. A cartridge containing a projectile, propellant and primer is chambered into the barrel, and when the trigger is pulled the firing pin strikes the cartridge, igniting the primer and propellant and pushing the bullet forward. Rifles are designed to be held in two hands and fired from the shoulder. There are several general main types—lever action, slide or pump action, and bolt action. The names essentially how the firearm operates to chamber and eject a cartridge, and cock the firing mechanism.

    In lever action rifles, the firing mechanism is cocked by dropping a lever located below the receiver (the receiver is simply the term used to describe the housing of a firearm’s operating parts) [19].  Lever action rifles were common historically but their popularity waned with the introduction of the pump-action rifles.

    firearms5.JPG

    Image courtesy of www.firearmsID.com, artwork by Erik Dahlberg.

    In bolt action rifles, the expended cartridge is extracted and ejected by drawing the bolt to the rear. Moving it forward strips a fresh cartridge from the magazine and inserts it into the firing chamber [20]. 

    firearms6.JPG

    Image courtesy of www.firearmsID.com, artwork by Erik Dahlberg.

    There are also semi-automatic rifles which, like semi-automatic pistols, use the energy of the fired ammunition to expel the empty cartridge, cock the firing mechanism and reload a fresh cartridge, or round.

    firearms7.JPG

    Image courtesy of www.firearmsID.com, artwork by Erik Dahlberg.

    In slide or pump action rifles, a moving slide is located beneath the barrel, and moved back manually to extract and eject the spent cartridge and cock the firing mechanism. Pushing the slide forward chambers a fresh cartridge. The magazine is located in a slim tube underneath the barrel [21].

    All rifle barrels are rifled—they contain the interior ridges that spiral down the length of the barrel to give the bullet spin and stabilization in flight.

    3.3 Shotguns

    For the layman, some of the biggest difference between shotguns and other firearms are 1) they usually fire different ammunition, typically pellet loads contained in shotshells and 2) they are smoothbore—in other words, the barrel of a shotgun does NOT contain rifling. The interior of a shotgun barrel is designed to be smooth so that nothing will deflect or slow the pellets as they travel along its length. The muzzle of a shotgun may also be constricted in order to produce a choke, which operates to keep the pellets grouped together for longer upon leaving the barrel. The influence the choke has on the shot pattern (shot patterns are very relevant in forensic firearms analysis) will increase with the distance of the shot—the range of a shotgun is short in comparison to that of a rifle, but the choke can improve the chances of hitting targets at near-to-mid ranges [22].

    The term gauge refers to the diameter of the shotgun barrel, but the gauge number is determined by the number of lead balls with the same diameter as the barrel that would, together, add to up a weight of one pound. For example, the actual measurement of the barrel diameter of a 20-gauge shotgun is 15.62mm, but the gauge is determined by the number of lead balls, with a diameter of 15.62mm, that would be required to make a total weight of one pound [23].

    Shotguns work in much the same way as rifles—the main difference is the smooth nature of the barrel. The most common action types are break-action shotguns and pump-action. They can also work in the same semi-automatic fashion as rifles.

    Break-action refers to a firearm where the barrels are hinged and can be opened to expose the breech and allow ammunition to be loaded. They are most common in double-barreled firearms.

    pumpaction.JPG

    Image courtesy of www.firearmsID.com, artwork by Erik Dahlberg.

    3.4 Automatic

    Automatic firearms are considered “automatic” because they will continue to fire so long as the trigger is depressed and ammunition is available. Automatic firearms are prohibited in Canada, and can only be possessed under a grandfathered licence.

    The most obvious and common types automatic firearms are machine guns and assault rifles (the oft-mentioned M16 and AK47 are both assault rifles).

    4.0 Forensic Analysis

    4.1 Goals and Purpose of Forensic Firearm Analysis

    The forensic study of firearms is based largely on the identification of bullets, cartridge casings or other ammunition component as having been fired from a specific firearm. Firearms identification is actually a form of tool mark identification, as the firearm—which is made of a material that is harder than the ammunition—acts as a tool to leave impressions or striations on the various components of the ammunition that come in contact with the firearm.

    Firearm examiners conduct a number of other examinations, in addition to the comparative examination of firearms and ammunition, including the detection of gunshot residue, examination of wounds, range determination, tool mark identification, serial number restoration and declaring a physical match of separated or broken items. Forensic firearm examiners will often attend crime scenes and autopsies to assist in determining the trajectory of projectiles, and the direction and range of shots [24].

    4.2 Bullet Examination

    One of the most common tasks of the firearm examiner is the microscopic examination of fired ammunition. There are several main goals to these examinations.

    First, microscopic examination of fired bullets can reveal patterns of striations caused by the bullet passing down the barrel of a firearm. Each firearm produces a unique set of striations, so firearm examiners can compare the bullets recovered from the scene with reference bullets fired from suspect firearms in order to see if the striations are the same. If they are, then the fired bullets can be declared to have originated from that specific firearm.

    Second, bullets can contain trace evidence as well as striations. For example, a bullet may pick up textile fibers, traces of paint or bits of wood, concrete or brick from intermediate targets (what the bullet passed through). A bullet might also acquire patterned markings, such as the impression of a metal window screen that the bullet had passed through [25]. 

    4.2.1 Identification by “Higher Order Class Characteristics”

    The interior workings and rifling patterns of an individual firearm produce unique marks on bullets and cartridge cases fired from that firearm. It is widely accepted that no two firearms produce the same marks—even those of the same make and model [26]. The machining and rifling created during the manufacturing process, combined with the use of the firearm, leave surface marks that are not reproducible in another firearm. These marks, especially striations produced from the bullet passing through the rifling in the barrel, are transferred to the bullets and casings when the firearm is discharged [27].

    6left.JPG

    Illustration of the internal rifling (lands and grooves) of a barrel.

    Image courtesy of FirearmsID.com

    Obviously, there is no practical method of comparing the striations on recovered bullets with the striations/rifling found on the interior surface of the barrel. Therefore, firearm examiners test whether or not a bullet was fired from the suspect firearm by creating reference bullets—bullets of the same make, style and caliber fired from the same suspect firearm under controlled laboratory conditions. In order to recover the reference bullets in as pristine a state as possible (minimal damage or deformation), the suspect firearm is discharged into something that will catch the bullet without damaging it—typically a water tank, although other methods are used as well. The reference bullet is then recovered, labeled and used as the control in comparison [28].

    The first examination of the questioned and control bullets is done with the naked eye or slight magnification. The goal of this initial examination is to determine the general rifling characteristics of the firearm that fired it. The characteristics are:

    1. caliber
    2. number of lands and groove (the ridges/valleys forming the rifling in the barrel)
    3. the direction of twist of the rifling
    4. the degree of twist of the rifling
    5.  the width of the lands and grooves [29].

    These are referred to as higher order class characteristics, and they allow the forensic firearms examiner to narrow the possible makes and models of firearms that could’ve fired the bullet [30] Many of these characteristics are found in databases, which make narrowing the field a faster process. Any deviations between the characteristics of the control bullets and the questioned bullets will immediately indicate to the examiner that the two bullets were fired from different barrels. However, if the characteristics appear to concur, then the next step is the microscopic comparison of the striations on the bullets [31].

    4.2.2 Microscopic Comparison

    The second stage of the examination is performed on a comparison stereomicroscope, which has special stages to allow the examiner to position both the questioned and control bullet in the same focal plane and rotate them on their long axis.

    Micro.JPG

    Firearms comparison microscope, which allows the examiner to view two objects simultaneously side by side. Image courtesy of firearmsID.com.

    The bullets are placed on the stages, and the control bullet is positioned so as to expose a land or groove with distinctive striations imprinted on its surface. Then, the questioned bullet is rotated until a land or groove comes into view with the same striation markings. The lands and grooves must have the same widths. More importantly, the two bullets must not be merely similar, but must have the same striation patterns with no significant differences [32]. Many firearm examiners require that identical patterns of three or more consecutive striations be found on each bullet in order to declare a match. However, it is not necessary that every striation pattern on the two bullets must match—particles of dirt in the barrel could make different markings, or the bullets could have entered the rifling in slightly different ways [33]. 

    4.png

    This image depicts the control bullet fired at the lab (left) and the questioned bullet collected from the crime scene (right), viewed under a comparison microscope.

    Determining a match accurately takes a significant amount of education, training and mentoring, due to the subtleties of bullet striations patterns. One must also take into account the fact that most bullets are not recovered from crime scenes in pristine state—they are often mangled, dirty and deformed. 

    Cartridge cases can also be examined for comparative analysis under the microscope. There are two basic types of tool marks found on cartridge cases—1) striated action marks, which are scratches produced from the cartridge case scraping laterally against the inner surface of the firearm, and 2) impressed action marks, which are created when some aspect of the firearm impacts against the case with enough pressure to leave an impression or indent, for example the impact of the firing pin. Striated action marks will usually only be found on firearms that eject expended cartridges, i.e. not revolvers. The comparison of cartridge cases follows the same process as with bullets—creation of a control or reference case to compare with the questioned case, and with databases.

    5.png

    The bases of the control and questioned cartridge cases, viewed under the comparison microscope. Image courtesy of firearmsID.com

    4.2.3 Potential Conclusions of the Examination

    One of the following three conclusions may be reached as a result of the comparison of bullets or cartridges [34].

    First, the questioned bullet or cartridge was fired in the submitted weapon.  This means that the class characteristics are consistent and the individual characteristics match.  This is a positive identification.

    1.png

    Example of a comparison yielding a positive identification. Image courtesy of firearmsID.com

    Second, the questioned bullet or cartridge was not fired in the submitted weapon.  This means that the class characteristics did not match - in this case no further microscopic examination would be done. This is a negative identification.

    2.png

    Example of a comparison yielding a negative identification. Image courtesy of firearmsID.com

    Third, the results of the microscopic comparison of the bullet or cartridge were inconclusive. This means that the class characteristics do match but that the individual characteristics found were insufficient to declare a match. The examiner does not give a negative identification in this case because the individual characteristics could have been altered either intentionally or through use.

    inconclusive2.jpg

    Example of a comparison yielding an inconclusive identification. Image courtesy of firearmsID.com

    Note:  Shot, the small pellets of lead that make up the most common shotgun projectile, cannot be matched back to a certain gun. This is because shotguns are smoothbore firearms, so the projectiles fired from shotguns do not get striations. However, spent shot shells, like cartridge casings, may have distinctive marks from ejection or the impact of a firing pin.

    4.2.4 Judicial Treatment

    The evidence from the microscopic comparison of bullets, cartridges cases and firearms has been long accepted in Canada. Published cases from as early as 1931 [35] and 1945 [36] accept the evidence of firearms experts in this field.

    4.3 Distance of Fire Determination

    Another common task of the forensic firearms scientist is the determination of the range and originating direction of the gunshot. This determination is based on a number of factors, but mianly the identification of gunshot residue (or GSR) and the examination of the wound and area surrounding it for distinctive shapes or patterns of burns, abrasions or the residue itself.

    4.3.1 Gunshot Residue and Wounds

    Firearms examiners are often called upon to estimate the range from which a gunshot was fired by examining the gunshot residue patterns on the victim’s skin or clothing. Gunshot residue consists of particles from the gun barrel or the surface of the bullet, and/or remnants of the propellant or primer. When the gun is fired, the residue from the shot is projected in a roughly conical cloud in the direction of the target—the larger the particle, the farther and straighter it will travel. Gunshot residue may also leak out or waft onto the shooter’s skin or clothing [37]. 

    The appearance of a gunshot wound, and the residue that may surround it, can indicate the range from which it was inflicted. Forensic pathologists usually place the range from which a gunshot wound was inflicted into one of four categories [38]:

    1. distant shots
    2. close-range shots
    3. near-contact shots
    4. contact shots (loose or tight)

    Distant shots are fired from such a range that no detectable GSR reaches the skin or clothing of the victim. There may, however, be a grayish ring around the wound composed of propellant combustion products and lubricant or metal form the bullet’s surface—this is referred to as a bullet wipe [39].

    1a.png

    A bullet wipe, seen on fabric.

    Close-range shots are inflicted at ranges that are short enough for GSR to reach the skin or clothing of the victim. Two types of GSR deposits are seen with these shots: 1) stippling (or tattooing) and 2) soot (smudging). The larger particles that produce stippling travel farther than the finer particles which make up the soot, so as the range of fire decreases the resulting GSR patterns go from widely dispersed stippling (farther range) to more concentrated stippling plus soot (closer range) [40].

     

    1b.png

    Examples of soot and stippling from contact to distant. Image courtesy of Murray Smith.

    A shot from near-contact range will produce stippling and smudging that is concentrated in a tight circle. The muzzle flash (the release of high temperature, high pressure gases which can be seen as a flash of light) may tear, burn or melt clothing fibers [41].

    1c.png

    At mid-range or near contact distance, most of the GSR particles are projected onto the skin. Image courtesy of Murray Smith.

    Loose-contact shots are fired with the gun’s muzzle just touching the target surface, and the muzzle flash will usually produce similar affect to those observed in near-contact shots. Tight-contact shots, particularly when the shot directly enters bone, often produce a characteristic jagged entrance wound caused by the gases from the propellant creating a pocket of hot gas between skin and bone. Particles of GSR will follow the bullet through the opening in the skin. This may also have the effect of blowing tissue back onto or into the weapon, and the hand of the shooter, providing trace biological evidence [42].

    1d.png

    On contact, particles of GSR will follow the bullet through the opening in the skin. Image courtesy of Murray Smith.

    4.3.2 Detecting and Using GSR

    As stated, the pattern of GSR on a target can be indicative of the distance from the muzzle of the firearm to the target. However, the patterning and density of the GSR will vary with the firearm and ammunition used. Therefore, the patterns are usually empirically generated, in a laboratory setting, by firing the questioned firearm (often on fabric or paper) to produce a succession of control patterns. These can then be compared with the questioned pattern noted on the victim, or on scene. However, duplication may not always be possible—although the examiner must make sure to use the same ammunition as that which produced the questioned pattern, unknown elements such as weather conditions or air currents at the time of initial discharge may have dispersed the GSR plume [43].

    GSR is not necessarily visible—the particles that make up the gas cloud are submicroscopic and invisible to the human eye—so firearms examiners use a number of different tests to detect its presence [44]. Testing is done using various procedures, including infrared photography [45], the Griess test [46], sodium rhodizonate [47] or—most commonly—a scanning electron microscope outfitted with an energy-dispersive spectrometer and imaging system(SEM/EDS). These machines show GSR as bright particles, which are then scanned for the presence of antimony, barium and lead—the three main components of GSR. These machines can be automated for unattended operation, although positive identifications still require human verification [48]. 

    1e.png

    SEM/EDS set-up. Image courtesy of Murray Smith.

    1f.png

    Exhibit that has been treated with sodium rhodizonate. The presence of lead is indicated by the pink reaction. Image courtesy of firearmsID.com.

    4.3.3 Relevance & Utility of GSR

    The examination of samples taken from the hands, face, or clothing of a shooting suspect can provide corroborative evidence linking the suspect to the discharge of a firearm. However, the evidentiary value of the results can be limited, depending on the circumstances. A positive result does not necessarily mean that the individual tested discharged a firearm—in some situations bystanders of victims may be more likely to display GSR deposits than the shooter. In addition, the examination of a GSR kit is very time consuming and thus expensive. The RCMP usually only analyzes kits that have the potential to provide evidence that cannot be determined by any other means—this is determined on a case by case basis [49]. 

    4.3.4 Shotguns and Range Determination

                The determination of the muzzle to target distance of a shotgun discharge is similar to the method used for other firearms, except is the pattern of pellet dispersal that is measured. As the pellets leave the barrel they begin to spread. The degree of spread is based on the distance they travel, and whether or not the shotgun in question has a choke in the barrel.

     

    1g.png

    Example of a control pellet pattern created for comparison purposes.

    4.4 Tool Marks

    Tool marks are the areas of damage produced when two surfaces come in contact. Whichever surface is softer will receive the majority of damage—the damage thus becomes the tool mark, and the harder surface is the tool. The general purpose of the discovery and examination of tools marks is to identify the type of tool that produced the marks, and then possibly identify a certain tool as the source of that mark [50].

    There are many potential sources for tool marks. Metal tools are manufactured in a number of fashions but most methods leave microscopic striations on their working surfaces. In addition, the mark made by a tool can have class characteristics that help identify what type of tool is it, or at least narrow the field of possible contenders—an axe, for example, leaves considerably different markings than a screwdriver [51]. This determination is generally easier when the tool mark left behind is a compression or cutting mark—sliding marks are usually just scratches and harder to identify in terms of class characteristics. The quality of the tool mark will also be greatly affected by its substrate—the material on which it was made. Soft metals, plastics and painted surfaces tend to be the best substrates, and hard metals or raw wood the worst [52].  Some examples of tool marks:

    • the marks made by firearms on fired bullets or cartridge cases
    • scrapes on a door frame or safe made by a screwdriver or a crowbar
    • cuts made by bolt cutters on a padlock or chain
    • mould marks on illegally manufactured drugs
    • damage resulting from a vehicle collision (paint transfer, impressions etc)
    • knife cuts in automobile tires, bone or cartilage
    • teeth marks of locking grip pliers on locks or doorknobs

    As with bullet and cartridge comparisons, it is generally not possible to compare the questioned tool directly to the tool mark it may have left. Instead, the examiner will commonly make control tool marks in the lab, which can then be compared with the questioned marks under a comparison microscope. Sometimes it will be necessary to make casts of both the control and questioned marks, and the substrate material is not practically examinable (i.e. a steering column) [53]. 

    1h.png
    1i.png

    The tool marks found the lock and surface of forced door can be matched, using the comparison microscope, to the tool in question—a flat head screwdriver. Images courtesy of Murray Smith.

    Tool marks can also be found in wounds and injuries, with bone as the typical substrate, and their class characteristics used to identify the tool that created them.

    1j.png
    1k.png

    In this case, the characteristics of the tool marks made by the suspected axe blade—viewed under the comparison microscope—match the tool marks imprinted on the skull. The axe was the weapon. Image courtesy of Murray Smith.

    As is the case with firearms examinations, tool mark examiners can reach one of three possible conclusions [54]

    4.4.1 Physical Match Determination

    One final use for the comparison microscope is the forensic examination of items that were at one point whole or adhered but have been physically broken or severed.

    1l.png
    1m.png

    A physical match found between two ends of a severed gas line, or an electrical cord. Images courtesy of Murray Smith.

     

    1 @ s.84(1) Criminal Code

    2 @s.84(3) Criminal Code

    3 @s.84(1) Criminal Code

    4 Max M Houck & Jay A Siegel, Fundamentals of Forensic Science (London: Elsevier/Academic Press, 2006) ch 21 @p 575ff [Houck & Siegel]

    5 Walter F Rowe, “Firearm and Tool Mark Examinations” in Stuart J James & Jon J Nordby, eds. Forensic Science: An Introduction to Scientific and Investigative Techniques” (Boca Raton: CRC Press, 2005) 391 @ 396ff [Rowe]

    Rowe, supra note 5 @p 397

    Houck & Siegel, supra note 4 @p 577

    R v TCM, 2007 BCSC 1778, 77 WCB (2d) 393, @p 47

    Rowe, supra note 5

    10 Rowe,supra note 5 @p 398

    11 Rowe @p 399

    12 Houck & Siegel, supra note 4 @p 577

    13 bid

    14 bid

    15 Rowe @p 392

    16 Ibid

    17 Ibid

    18 Ibid

    19 Howe @p 393

    20 Ibid

    21 Ibid

    22 Houck and Siegel, @p 573

    23 Houck and Siegel, @p 574

    24 Murray Smith, “Firearms and Tool Marks” (Lecture delivered at the Faculty of Law, University of Ottawa, 2011), [unpublished] [Smith]

    25 Howe @p 402

    26 Houck and Siegel  @p 582

    27 Ibid

    28 Houck and Siegel @p 583

    29 Supra note 25

    30 Ibid

    31 Houck and Siegal @p 583

    32 Ibid

    33 Howe @p 404

    34 Howe@p 405

    35 Rex v Nahirniak, [1931] 2 WWR 604 @para 14 (CA)

    36 R v Prince, [1945] BCJ No 15 @para 25 (CA)

    37 Howe @p 409

    38 Ibid

    39 Houck and Siegel @p 589

    40 bid

    41 bid

    42 Howe @p 410

    43 bid

    44 Houck and Siegel @p 588-591

    45 Infrared photography—used on dark or blood-soaked clothing. Elements of GSR absorb infrared radiation, therefore enable detection by the human eye.

    46 Griess test—the GSR is transferred onto photographic paper that reacts with the nitrates in GSR to form a dye and thus a visible pattern.

    47 Sodium rhodizonate is a liquid solution that can be sprayed on GSR patterns that have been transferred to paper. The lead within the residue reacts with the spray and turns a bright pink or purplish-blue. Lead-free ammunition does exist.

    48 Houck and Siegel @p 593

    49 Smith, supra note 1

    50 bid

    51 Houck and Siegel @p 594

    52 Howe @p 414-415

    53 Houck and Siegel @p 595

    54 Howe at 416-417