Ballistic fingerprinting refers to a set of forensic techniques that rely on marks that firearms leave on bullets to match a bullet to the gun it was fired with. It is a subset of forensic ballistics (the application of ballistics to legal questions) and internal ballistics (the study of events between the firing of a gun and the bullet leaving the barrel).
Rifling, which first made an appearance in the 15th century, is the process of making grooves in gun barrels that imparts a spin to the projectile for increased accuracy and range. Bullets fired from rifled weapons acquire a distinct signature of grooves, scratches, and indentations which are of value for matching a fired projectile to a firearm.
The first firearms evidence identification can be traced back to England in 1835 when the unique markings on a bullet taken from a victim were matched with a bullet mould belonging to the suspect. When confronted with the damning evidence, the suspect confessed to the crime. Alexandre Lacassagne was the first scientist to try to match an individual bullet to a gun barrel.
The first court case involving firearms evidence took place in 1902 when a specific gun was proven to be the murder weapon. The expert in the case had read about firearm identification, and had a gunsmith test-fire the alleged murder weapon into a wad of cotton wool. A magnifying glass was used to match the bullet from the victim with the test bullet.
Calvin Goddard, physician and ex-army officer, acquired data from all known gun manufacturers in order to develop a comprehensive database. With his partner, Charles Waite, he catalogued the results of test-firings from every type of handgun made by 12 manufacturers. Waite also invented the comparison microscope. With this instrument, two bullets could be laid adjacent to one another for comparative examination.
In 1925 Goddard wrote an article for the Army Ordnance titled "Forensic Ballistics" in which he described the use of the comparison microscope regarding firearms investigations. He is generally credited with the conception of the term "forensic ballistics," though he later admitted it to be an inadequate name for the science.
Ballistic fingerprinting techniques are based on the principle that all firearms have inevitable variations due to marks left by the machining process, leaving shallow impressions in the metal which are rarely completely polished out. Also, normal wear and tear from use can cause each firearm to acquire distinct characteristics over time.
The simplest considerations are the gross differences. A 10 mm bullet, for example, could not have been fired from a 9 mm barrel.
When a bullet is fired through a rifled barrel, the raised and lowered spirals of the rifling etch fine grooves called "striations" into the bullet. These can be matched with the barrel through which the bullet was fired. Examiners distinguish between striations common to all guns of a particular type ("class characteristics") and those unique to a particular gun ("individual characteristics").
The class characteristics depend upon the type of rifling in the barrel, which varies among manufacturers and models in number and shape of the grooves, twist rate, and direction. Colt, for example, traditionally uses a left-hand twist, while Smith & Wesson uses a right hand twist; a current production M16 rifle uses a 1 in 7 inch twist, while most civilian AR-15s and the current Mini-14 use a 1 in 9 inch twist. Marlin Firearms use a distinctive 16-groove Micro-Groove rifling in many of their firearms, while the M1903 Springfield rifle had two, four, or six grooves depending on the manufacturer. Polygonal rifling may leave striations that are difficult to match to a particular barrel.
Individual characteristics are caused by imperfections in the rifling process and tools, but also by the wear and tear caused by regular use, and can therefore change over time. Criminals or those concerned with government intrusion in privacy sometimes attempt to alter a gun's individual characteristics by changing or shortening the barrel, or by rubbing its interior with a steel brush.
Marks on the cartridge case can be matched to marks in the chamber and breech. For a number of reasons, Cartridge cases are often easier to identify than bullets. First, the parts of a firearm that produce marks on cartridge cases are less subject to long-term wear, and second, bullets are often severely deformed on impact, destroying much of the markings they acquire.
Ballistic fingerprinting of bullets does not work with firearms such as shotguns that fire shot-containing cartridges. In many cases the shot rides inside a plastic sleeve that prevents it from ever touching the barrel, and even in cases where the shot does touch the barrel, the random movement of the shot down the barrel will not leave any consistent marks. But shotgun cases can still be examined for firing pin marks and the like.
Ballistic fingerprinting aids
Some localities, particularly Maryland, have attempted to build up a large database of "fingerprints"; in the case of the Maryland law, all new firearms sales must provide a fired case from the firearm in question to the Maryland State Police, who photograph it and log the information in a database. The Maryland State Police wrote a report critical of the program and asking the Maryland General Assembly to disband it, since it was expensive and had not contributed to solving a single crime. Subsequently however, the database did provide evidence used to obtain one murder conviction at an estimated cost of 2.6 million dollars per conviction.
A California Department of Justice survey, using 742 guns used by the California Highway Patrol as a test bed, showed very poor results; even with such a limited database, less than 70% of cases of the same make as the "fingerprint" case yielded the correct gun in the top 15 matches; when a different make of ammunition was used, the success rate dropped to less than 40%.
There have been several proposals for the mandated marking of bullets to aid in ballistic fingerprinting, and some jurisdictions have passed legislation to that effect. California, for instance, passed a bill AB 1471 which requires all new models of handguns to be equipped with microstamping technology by 2010. However, due to the way the California law was written, microstamping cannot be implemented currently due to patent encumberment.
Several techniques have been proposed:
- Firearm microstamping is a process that engraves the make, model, and serial number on the cartridge and on the face of the firing pin, which stamps the primer as the firing pin impacts it.
- A British researcher proposed in a 2008 report that ammunition manufacturers coat their bullets with pollen, or with a pollen deposit coated with a metal oxide. Pollen grains are sticky enough and have a sufficiently hard outer case to survive being fired. They also attach themselves to the clothing and hands of people who handle the ammunition and the gun, providing an additional forensics clue (the pollen is extremely difficult to wash off completely, according to the researchers). If manufacturers used unique pollen varieties or unique mixtures of pollen and oxide coatings, the manufacturing database could be used to quickly identify a bullet found at a crime scene, assuming the investigating bodies equip themselves with the necessary pollen-identification equipment.
- John Lott article on Maryland ballistic fingerprinting
- Copy of California DOJ Study of ballistic fingerprinting effectiveness
- "Scientists reject bullet-mark database" MSNBC Deep Background, Pete Williams, NBC News Justice Correspondent, Posted on Wednesday, March 05, 2008
- Kopel, David B. (2008). "Ballistic fingerprints". In Ayn Embar-seddon, Allan D. Pass (eds.). Forensic Science. Salem Press. p. 109. ISBN 978-1-58765-423-7.
- Kopel, 110.
- "Maryland State Police Report Recommends Suspending Ballistics ID System", Washington Post, January 18, 2005
- "Ballistics Database Yields 1st Conviction", Washington Post, April 2, 2005
- Engineering & Physical Sciences Research Council (EPSRC), a British research funding agency
- Newscripts, Chemical & Engineering News, 86, 33, 18 Aug. 2008, p. 88