“A discipline of forensic science which has as its primary concern to determine if a bullet, cartridge case or other ammunition component was fired by a particular firearm,” as defined by the Association of Firearm and Toolmark Examiners (AFTE).
On May 2, 1863, while returning from a scouting mission during the Battle of Chancellorsville, Virginia, Confederate General Thomas J. “Stonewall” Jackson was fired upon and wounded. Dr. Hunter McGuire removed a spherical ball 0.675 in. in diameter from Jackson's right hand and amputated his left arm. Although he survived the wounding, he developed pneumonia and died on September 10, 1863. Because the Union Army had abandoned the use of the smooth-bore musket that fired this type of ammunition the previous year, investigators were able to conclude that Jackson had been shot by his own troops, who were still using that firearm. Many believe this to be the first recorded instance of the use of firearms identification for forensic purposes. See also: Criminalistics
Forensic firearms identification is based on the theory that the methods used in the production of a firearm create random imperfections on the metal surfaces of that firearm which, in sufficient quantity and quality, create a unique pattern for that particular weapon. For example, virtually all firearm barrels start out as a solid steel rod. This rod is drilled to the approximate final diameter of the barrel, and then reamed using another tool to smooth out the interior of the barrel. The barrel is then rifled, a process that cuts or forms grooves, which spiral down the barrel. Next, the barrel is given a final smoothing. All of these processes require the use of tools that are, by their very nature, harder than the metal of the barrel. Any imperfections that are present on the tools as a result of their manufacture or subsequent wear will be randomly transferred to the surface of the barrel as marks, nicks, or scratches. Because these marks, known as striations or striae, are the result of the random wear of the tool and their random placement in the barrel, they create a unique pattern within that barrel (Fig. 1). Similar markings can be found on a firearm's firing pin, breech face (rear of the frame), and cartridge-case extractor and ejector; within the chamber of the firearm; and on any other metal surface that has been machined or formed.
Modern ammunition (the cartridge or shotgun shell) generally consists of one or more projectiles (bullet or shot) contained in a cartridge case that also contains a predetermined amount of gunpowder. In the case of rifles and handguns, the bullet is inserted into one end of the cartridge case. In shotguns, small balls of lead, steel, or some other metal are contained within the case. The gunpowder is contained within the cartridge case in virtually all forms of ammunition. At the opposite end of the cartridge case, a contact explosive, known as a priming mixture, is either pressed into a cup (center fire ammunition) or spun into the rim of the cartridge case (rimfire ammunition). When discharging a firearm, pulling the trigger causes the weapon's firing pin to strike the rear of the cartridge case, detonating the priming mixture, which in turn ignites the gunpowder. As the gunpowder burns, the gases produced cause the pressure to build up within the cartridge. This pressure very quickly reaches the point where the bullet is forced out of the end of the cartridge case and into the barrel of the firearm. As the pressures continue to mount, the bullet, which is slightly larger than the diameter of the barrel, conforms to the rifling in the barrel; that is, as it is forced down the barrel, the bullet is in intimate contact with the surface of the barrel. At the same time, the cartridge case is slammed back into the breech face of the firearm.
This intimate contact of the bullet and cartridge case with the firearm results in the striations on the various parts of the firearm being impressed into the bullet and cartridge case. In essence, a “signature” of the barrel, breech face, and other machined parts of the firearm are imparted to the surface of the ammunition components (Fig. 2).
These signatures can be used by an expert to form an opinion as to whether a particular ammunition component has been fired from a particular firearm. This is accomplished by a direct comparison of the surface of the fired bullet or cartridge case to the surface of a fired bullet or cartridge case (known as a reference bullet or reference cartridge case) that has been fired from a known firearm. This comparison is dependent on the ability to produce reference material that ideally possesses only surface markings obtained as a result of having been cycled through the particular firearm in question. Reference bullets can be obtained by firing the weapon in question into some nonabrasive material dense enough to stop the bullet but not so dense as to damage it. Originally, this material consisted of rolls of absorbent cotton, or a soft grade of cotton waste. Although still in use in some places today, most laboratories now obtain reference bullets by firing into a tank filled with water. These tanks may be either vertical or horizontal and contain enough water to slow and stop the bullet's travel without damaging the surface.
Once the reference material has been recovered, the direct comparison is accomplished by using a comparison microscope (Fig. 3), a device that consists of two microscope bodies connected by an optical bridge, allowing the user to view portions of each microscopic field as if they were placed side by side. Although the comparison microscope has been known in one form or another since about 1886, it wasn't until 1925 that Philip O. Gravelle produced the forerunner of the modern firearms comparison microscope. This device's usefulness as an analytical tool became apparent in 1929, when it was used by Colonel Calvin Goddard in the evaluation of evidence from the St. Valentine's Day Massacre in Chicago, Illinois. Since then, although there have been advances in optics and design, the basic use of the firearms comparison microscope has not changed. For analysis, the surfaces of reference material and evidence are directly compared to one another in an effort to evaluate the correspondence or noncorrespondence of marks produced as a result of the firing process (Fig. 4). See also: Forensic microscopy; Optical microscope
Today, forensic firearms examination encompasses much more than a determination of whether ammunition components were fired from a particular firearm.
Chemical analysis for the presence of gunshot residues
The process of discharging a firearm produces a number of residues, both visible and invisible, which can provide valuable information to the forensic firearms examiner. The detonation of the priming mixture results in the vaporization of the metallic components in the mixture. These vapors are blown out of any openings in the firearm, including the areas where the trigger and hammer are inserted into the frame, the cylinder gaps in revolvers, the ejection port in autoloaders, and the muzzle of the firearm. As these vapors cool, they form small spherical particles that contain combinations of the various metals present, particularly lead, antimony, and barium, which will deposit on any surface within a few feet of the firearm. Lead, antimony, and barium, while not particularly rare individually, are not generally found together in nature. As a result, their presence in a sample is indicative of gunshot residue. Although the presence of lead, antimony, and barium in a sample is not definitive proof of gunshot residue, their presence can be of investigative importance, particularly if these residues are found on samples taken from the hands of someone suspected of having discharged a firearm. Samples can be taken from any surface suspected of having gunshot residues on it either by swabbing the area with a cotton-tipped swab wet with dilute nitric acid, or by dabbing the area with an aluminum disc layered with adhesive. The samples are then analyzed by atomic absorption spectrophotometry (the cotton-tipped swabs) or scanning electron microscopy with energy dispersive x-ray analysis (the adhesive discs). Scanning electron microscopy has the added advantage of providing a visual confirmation of the particle's size and shape in addition to its elemental composition. See also: Atomic spectrometry; Scanning electron microscope
The incomplete combustion of gunpowder produces gaseous residues that are rich in nitrates and nitrites as well as particles of partially burned gunpowder. As these residues exit the muzzle of the firearm, they spread out in the atmosphere as they travel forward away from the muzzle. Depending on the type of firearm, residues can travel 8–10 ft (2.4-3 m) or more from the muzzle. The farther away from the muzzle the residues are, the more spread out they will be. Knowing this, a muzzle-to-target distance can be approximated by examining any residues present around a bullet entrance hole and comparing the density of the residues and the amount of spread to a series of targets fired from varying distances using the same firearm and ammunition. The visible residues, consisting mainly of unburned and partially burned gunpowder particles, are examined using either a lighted magnifier or a stereomicroscope. Most of the time this is sufficient for an estimation of the range of fire to be made; however, many laboratories will also chemically process the target in order to visualize the pattern formed by the combustion products, mainly nitrates and nitrites, which have also been deposited. Most of the common tests used for this purpose rely on the formation of colored diazo compounds (dyes).
Serial number restoration
Since 1968, virtually all firearms manufactured in, or imported into, the United States have serial numbers stamped on them, generally somewhere on the frame. Often, in an effort to impede the tracing of a firearm's ownership, the serial number will be obliterated in some way, such as by grinding, filing, or punching.
Most serial numbers on firearms are created by stamping the number into the metal with considerable force. This process deforms the structure of the metal underneath the stamped number. If, at some later time, the numbers are removed by filing or grinding, the deformed metal that was underneath will generally remain. Because the deformed metal is under stress, if the proper acid solution is applied the deformed metal will dissolve at a faster rate than the surrounding metal, revealing the removed numbers. Although there are many solutions that can be used for this purpose, the most commonly used consist of a mixture of dilute hydrochloric acid and copper chloride.
National Integrated Ballistic Information Network
In 1999, the Bureau of Alcohol, Tobacco, and Firearms established the National Integrated Ballistic Information Network (NIBIN), a database of ballistics imaging technology. Participants in this program acquire digital images of the markings made on fired bullets and cartridge cases using the Forensic Technology Integrated Ballistic Identification System (IBIS) and then upload them to the NIBIN database, where they are electronically compared to earlier entries. If a high-confidence match occurs, firearm examiners compare the original evidence using a comparison microscope to make the final decision regarding whether the items actually do match. Using this system, otherwise unrelated cases can be associated with one another.