Researchers at the National Institute of Standards and Technology are working to improve ballistics matching methods with assistance from the Prince George's County, Maryland, Police Department Crime Laboratory. Their work together will contribute to a collection of topographic data from thousands of fired bullets and cartridge cases.
The collection, which they ultimately plan to issue as an open research database, will improve the scientific basis of forensic techniques used to match bullets and cartridge cases to specific firearms. The biggest potential improvements in firearms identification will come from the ability to measure the degree of similarity between marks left on bullets and cartridge cases by different firearms and the variability of marks made by an individual firearm.
"That's the holy grail in firearms and tool marks identification," explains Robert M. Thompson, a senior forensic science research manager in the NIST Office of Special Programs. To meet the challenge, forensic scientists need detailed measurements at the microscopic scale to map the ridges, grooves and other impressions that various firearms leave on bullets and cartridge cases.
The researchers are mapping the surfaces of bullets fired from gun barrels that were machined consecutively. Firearm parts manufactured consecutively – machined in order, one after another – have the greatest similarity, and therefore produce tool mark patterns with the greatest similarity. Such impression patterns represent the greatest challenge for forensic firearms analysis, because they present the best chance for an erroneous identification. Researchers must measure and compare features around a millionth of a meter to fully understand the differences in patterns made by consecutively manufactured gun barrels.
If a pattern matching algorithm can distinguish between tool mark patterns on bullets fired from consecutively manufactured barrels, then investigators can trust it to accurately assess whether bullets and cartridge cases from a crime scene match test rounds fired from a specific firearm barrel. The database will foster the development and validation of advanced algorithms, mathematical similarity criteria, and quantitative confidence limits. The key ingredient of the database will be highly accurate NIST topographic measurements of bullets and cartridge cases, many from consecutively manufactured firearms, which is where the Prince George's County Crime Laboratory is lending a hand.
A firearms examiner from the police department's crime lab learned about the NIST database project at a recent Association of Firearm and Tool Mark Examiners meeting. The Prince George's County crime lab had acquired 30 consecutively manufactured 9-mm caliber pistol barrels and volunteered to let NIST borrow them in order to collect more data.
The NIST team took the barrels to a test firing range at the Bureau of Alcohol, Tobacco, Firearms and Explosives. They attached each barrel to a weapon and fired it into a water tank several times. The water slows the bullet and helps to preserve the tool marks from the barrel. The researchers then collected the bullets in almost pristine condition.
"We're grateful to our colleagues in Prince George's County," says Thompson. "The data we obtain from bullets fired through their barrels is essential to the success of our database project. Obtaining a large amount of data from a wide range of firearms and ammunition will be the key to making it a success."
On Aug. 1, the Prince George's County Police Crime Lab director and examiners visited NIST and toured the ballistic measurement laboratories to see how NIST is measuring bullet and cartridge case topographies. Read more about the database project in the latest issue of NIST Forensic Sciences News, online at www.nist.gov/forensics/upload/Summer-2014.pdf (link opens PDF).