Forensic examination of firearms

Firearms

Forensic examination of firearms was established in the United States and Europe during the early years of the 20th century. In many laboratories, firearms examiners are also involved in toolmarks, gunshot residue, chemical analysis of bullets, serial number restoration, and distance determinations of how far a shooter was from the victim or target. Although the term "forensic ballistics" is sometimes used to describe this work, it is not a correct description. Ballistics is the study of the motion and trajectory of projectiles, while firearms analysis focuses on the study of bullets, cartridge cases, and other materials associated with firearms as physical evidence.

Types of Firearms

Pistols or handguns, rifles, shotguns, machine guns, automatic weapons, homemade firearms, and hobby guns can all be encountered by firearms examiners. Pistols are smaller guns designed to be fired with one hand and include the revolver and semiautomatic pistols, often erroneously called "automatics." True automatics fire continuously as long as the trigger is pulled. Semiautomatic guns exploit gas pressure and springs to eject the spent cartridge, load a new one, and cock the weapon for the next shot, but a separate trigger pull is required to fire the next cartridge. Homemade weapons are usually small handguns that carry few or one shot and have been referred to as "zip guns" and Saturday Night Specials. Hobbyists such as those that reenact Civil War battles use smooth bore guns with round projectiles (ball shot) and black powder, and so a forensic firearms examiner must be familiar with such weapons.

Analysis

Tasks of firearms investigators are similar to those of other forensic analysts in that they seek class characteristics and characteristics that will individualize evidence by linking a specific weapon to a specific bullet or cartridge. Modern guns all have rifled barrels, meaning that a series of grooves (lands and grooves) are machined into the barrel in a spiraling pattern. When the bullet is forced over these lands and grooves, spin is imparted to the bullet, which stabilizes the trajectory and greatly increases accuracy over smooth bore weapons. This contact creates distinctive striation patterns on the bullet that can later be matched to the weapon. The striations are usually unique to one gun based on the way the gun is manufactured and used. To machine lands and grooves into a barrel, different tools can be used, all of which involve metal-to-metal contact. Although a tool is used to cut many barrels, each barrel that is machined will cause wear on the tool, creating small irregularities and random wear patterns that can be transferred to the next barrel machined. This next barrel will also create irregularities and so on, meaning that the markings on each new barrel cut will be slightly different and thus unique. In addition, once a gun is purchased, it will have a different usage pattern from every other gun and thus a unique wear pattern on the lands and grooves. Thus, slight differences created during milling combined with individualized wear patterns lead to a unique striation pattern for most guns. As a result, the marks each gun imprints on bullets and cartridge cases should be unique or very nearly so. These imprints are a kind of impression evidence, as are toolmarks. For this reason, many firearms examiners are also responsible for the analysis of toolmark evidence.

The classifications associated with firearms evidence include the caliber of a weapon or ammunition, the number of lands and grooves, the direction and degree of twist in the rifling pattern, and the make and manufacturer of a weapon or ammunition. Thus, when evidence is submitted such as a bullet or case, the examiner will attempt to identify as many characteristics as possible. The same approach would be used if a weapon were submitted. When a weapon as well as bullets and/or casings are available, then the examiner can attempt to prove or disprove the theory that the suspect weapon fired the bullet and casing. The primary tool used to do so is the comparison microscope, which allows for a side-by-side comparison of a bullet or casing submitted as evidence (for example, if a bullet was recovered at autopsy and a casing was found at a crime scene) with bullets or casings obtained from test firings of the weapon. To obtain these standards, the firearms examiner discharges the suspect weapon into a trap of some kind (a water tank or boxes filled with cotton or oil-soaked cotton) using the same type of ammunition as the submitted evidence. By firing into the trap, all of the striations on the bullet are preserved and can be used for comparison to the submitted bullets. If the striations on the test bullet or casing match up with those of the evidence, then the same weapon was used to fire both.

The handling of firearms evidence is driven first by safety concerns. If a weapon is recovered at a scene, it is normally unloaded, but only after complete and careful documentation. Information such as how many bullets were fired, which chamber was under the hammer (in the case of revolvers), and general condition of the weapon can be crucial. Any marking of guns, bullets, or casings is done carefully to avoid damaging the striations. If bullets are imbedded in walls, floors, or other materials, it is usually preferable for entire sections of the material containing the bullet to be removed and transported to the lab, where the examiner can carefully remove the projectile without damaging it. Conversely, the examiner will study the evidence carefully, looking for latent fingerprints and other trace evidence such as hair, blood, and tissue on or in the barrel or impressions such as textile prints on bullets. All must be carefully documented and preserved before the actual firearms examination can begin.

History

Firearms examination had become widely accepted by the 1930s. The first reported use of a comparison microscope came in 1922, with increasing use of bullets and cartridge cases as evidence. In the early 1920s, Charles Waite became involved as an investigator in a case in which Charles Stielow was convicted of the murder of his landlord and housekeeper based primarily on testimony related to firearms. An expert at the trial stated that Stielow's pistol fired the fatal shots, and Stielow was sentenced to die. Shortly thereafter, another man confessed to the crime, prompting a reexamination. No evidence of a conclusive match of the bullet to Stielow's pistol was found. As a result of his involvement in the case, Waite collected extensive information about the rifling characteristics of firearms in the United States and Europe. In 1924, he started a private laboratory called the Bureau of Forensic Ballistics staffed by pioneers such as Calvin Goddard, John Fisher, and Philip Gravelle. Waite left shortly after that, while Gravelle became a strong advocate of the use of the comparison microscope. Fisher developed an instrument called a helixometer used to measure the angle of twist of the rifling, and Goddard wrote extensively about firearms examination. Goddard became involved in a pivotal early firearms case, the Sacco-Vanzetti trial that had taken place in 1922 and resulted in convictions and sentences of death for the defendants. Goddard joined in a review of the case in 1927.

On April 20, 1920, a robbery took place in South Braintree, Massachusetts, in which five men robbed a paymaster, killing him and the guard. A month later, two men, Sacco and Vanzetti, were charged with the crime. The case took a political twist since both men were of foreign descent and were anarchists, a movement that was spreading across the United States and Europe at the time. A .38 pistol was recovered from Vanzetti, but it could not be conclusively tied to any of the evidence recovered. From the bodies, .32 ACP bullets were recovered, and Sacco had a .32 pistol in his possession when arrested. Many experts testified about the firearms evidence while riots and protests broke out. The men were convicted, and, during his review, Goddard was able to link one cartridge case and a fatal bullet to Sacco's gun. The executions took place seven years after the crime, on August 23, 1927. Although controversial at the time, later firearms examination supported Goddard's findings.

In 1929, Goddard became involved in the famous St. Valentine's Day Massacre and was able to show that the murdered men had been killed by two Thompson submachine guns. His work in the case was well received, and shortly thereafter Goddard left the Bureau of Forensic Ballistics to direct the new Scientific Crime Detection facility in Chicago that would eventually become the Chicago Police Department Laboratory. During this period, developments in Europe were paralleling those in the United States, led by men such as Victor Balthazard. Balthazard was the first in Europe to identify fabric impressions of textiles on bullets and to use firing pin impressions, breechblock marks, and extractor/ejector marks. The study of such impressions employing a comparison microscope is still the primary approach used.

Other tasks undertaken by firearms examiners include the restoration of obliterated serial numbers from guns or other metal objects, distance determinations, reconstruction of shooting events, and, occasionally, the chemical characterization of bullet fragments when the damage prevents the standard microscopic approach. The most well-known example of this was conducted on the bullets involved in the assassination of President John F. Kennedy in 1963. In that case, neutron activation analysis (NAA) was applied to bullets and fragments in an attempt to determine how many bullets were represented by the recovered pieces. As in all areas of forensic science, computers and computer databases are becoming increasingly important. The National Integrated Ballistics Information Network (NIBIN) is coordinated by the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

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