Handguns predominated as the firearm of choice in shooting-related crimes, especially in the United States of America. As with most firearms, the fired ammunition components may acquire sufficient unique and reproducible microscopic marks to be identifiable as having been fired by a single firearm. Making these comparisons is correctly referred to as firearms identification, or sometimes called as “ballistics”. Historically, and currently, this forensic discipline ultimately requires a microscopic side-by-side comparison of fired bullets or cartridge cases, one pair at a time, by a forensic examiner to confirm or possibly eliminate the two items as having been fired by a single firearm. For this purpose, the traditional tool of the firearms examiner has been what is often called the ballistics comparison microscope. Comparison microscope is essential for the forensic ballistics expert in comparing bullets and spent cartridge casings.

The interior of a gun’s barrel is machined to have grooves that force the bullet to rotate as it travels along it. These grooves and their counterpart, called “lands” imprint groove and land impressions on the surface of the bullet. Together with these land and groove impressions, imperfections on the barrel surface are incidentally transferred to the bullet’s surface. Because these imperfections are randomly generated, during manufacture or due to use, they are unique to each barrel. These patterns or imperfections, therefore, amount to a “signature” that each barrel imprints on each of the bullets fired through it. It is this “signature” on the bullets imparted due to the unique imperfections on the barrel that enable the validation and identification of bullets as having originated from a particular gun. Comparison microscope is used to analyze the matching of the microscopic impressions found on the surface of bullets and casings.

When a firearm or a bullet or cartridge case are recovered from a crime scene, forensic examiners compare the ballistic fingerprint of the recovered bullet or cartridge case with the ballistic fingerprint of a second bullet or cartridge case test-fired from the recovered firearm. If the ballistic fingerprint on the test-fired bullet or cartridge case matches the ballistic fingerprint on the recovered bullet or cartridge case, investigators know that the recovered bullet or cartridge case was also fired from the recovered gun. A confirmed link between a specific firearm and a bullet or cartridge case recovered from a crime scene constitutes a valuable lead, because investigators may be able to connect the firearm to a person, who may then become either a suspect or a source of information helpful to the investigation. Forensic innovator Calvin Goddard offered ballistic identification evidence in 1921 to help secure convictions of accused murderers and anarchists Nicola Sacco and Bartolomeo Vanzetti. On April 8, 1927, Sacco and Vanzetti were finally sentenced to death in the electric chair. A worldwide outcry arose and Governor Alvin T. Fuller finally agreed to postpone the executions and set up a committee to reconsider the case. By this time, firearms examination had improved considerably, and it was now known that an automatic pistol could be traced by several different methods if both bullet and casing were recovered from the scene. Automatic pistols could now be traced by unique markings of the rifling on the bullet, by firing pin indentations on the fired primer, or by unique ejector and extractor marks on the casing. The committee appointed to review the case used the services of Calvin Goddard in 1927. Goddard used Philip Gravelle’s newly-invented comparison microscope and helixometer, a hollow, lighted magnifier probe used to inspect gun barrels, to make an examination of Sacco’s .32 Colt, the bullet that killed Berardelli, and the spent casings recovered from the scene of the crime. In the presence of one of the defense experts, he fired a bullet from Sacco’s gun into a wad of cotton and then put the ejected casing on the comparison microscope next to casings found at the scene. Then he looked at them carefully. The first two casings from the robbery did not match Sacco’s gun, but the third one did. Even the defense expert agreed that the two cartridges had been fired from the same gun. The second original defense expert also concurred. The committee upheld the convictions. In October 1961, ballistics tests were run with improved technology using Sacco’s Colt automatic. The results confirmed that the bullet that killed the victim, Berardelli in 1920 came from the same .32 Colt Auto taken from the pistol in Sacco’s possession. Subsequent investigations in 1983 also supported Goddard’s findings.

Colonel Goddard was the key forensic expert in solving the 1929 St. Valentine’s Day Massacre in which seven gangsters were killed by rival Al Capone mobsters dressed as Chicago police officers. It was also led to the establishment at Northwestern University, United States’ first independent criminological laboratory, which Goddard headed, and where ballistics, fingerprinting, blood analysis and trace evidence where brought under one roof. In 1929, using a dissecting microscope adapted for the ballistics comparison by his partner, Phillip Gravelle, Goddard used similar techniques to absolve the Chicago Police Department of participation in the St. Valentine’s Day Massacre. The case of Sacco and Vanzetti, which took place in Bridgewater, Massachusetts, is responsible for popularizing the use of the dissecting microscope for bullet comparison. Forensic expert Calvin Goddard’s conclusions were upheld when the evidence was reexamined in 1961.