ForensicsNIST releases 3D ballistics research database

It is a staple of the TV-crime drama: a ballistics expert tries to match two bullets using a microscope with a split-screen display. One bullet was recovered from the victim’s body and the other was test-fired from a suspect’s gun. If the striations on the bullets line up — cue the sound of a cell door slamming shut—the bad guy is headed to jail.

In the real world, identifying the firearm used in a crime is more complicated. However, the basic setup is correct. Ballistics examiners match bullets visually, and they have been doing it this way for almost 100 years. But testimony based on visual examination leaves out something important.

“When an expert testifies that two bullets are a match, the jury wants to know, ‘How good a match is it?’” said Xiaoyu Alan Zheng, a mechanical engineer who conducts forensic science research at the National Institute of Standards and Technology (NIST). “No forensic results have zero uncertainty.

”Researchers are developing statistical methods for quantifying that uncertainty, and the main obstacle they face is a lack of sufficient data. NIST says that this month NIST released the largest open-access database of its kind — the NIST Ballistics Toolmark Research Database — to help remove that obstacle.

Led by Zheng, this database effort is partly in response to a 2009 report from the National Academy of Sciences, which highlighted the need for statistical methods to estimate uncertainty when matching ballistic and other types of forensic pattern evidence. The development of the database was largely funded with a grant from the National Institute of Justice.

Sources of uncertainty
When matching a bullet to a gun, examiners look at striations that are carved into the bullet by rifling in the gun’s barrel. If the cartridge case is left behind, they can also look at impressions left on it by the weapon’s breech face and firing pin.

But these clues can sometimes be misleading. For instance, two gun barrels that are manufactured consecutively may produce bullets with very similar markings. That can lead to false matches. On the other hand, a gun might change over a short time due to wear on the parts or accumulation of debris in the barrel. If that happens, a single firearm might produce bullets that look like they were fired from different guns.

These confounding factors introduce uncertainty into examination results. Researchers would like to quantify this uncertainty using statistical methods, and to