Do bullets tell tales?
The ballistics expert left no room for doubt. Testifying November 6 in the trial of accused sniper suspect John Allen Muhammad, he asserted that bullets from victims' bodies matched the rifle found in Muhammad's Chevrolet Caprice "to the exclusion of all other firearms." But the same day, in a Baltimore courtroom, ballistics itself was on trial. There, a federal judge agreed to examine the scientific foundations of bullet matching before deciding whether an expert's assertion of a "match" could be admitted in an upcoming murder trial.
That hearing is just the most recent challenge to bullet "fingerprinting." Forensic science looks little short of miraculous on television shows like CSI, but in the real world, old-time techniques like fingerprint and handwriting analysis have come under fire from critics who say they don't meet the same scientific standards as newer tools like DNA tests. Now, defense lawyers and scientists are leveling the same charge at two mainstays of bullet matching: lead analysis, which links bullets based on chemical traces in the metal, and ballistics, which relies on the distinctive marks that a gun leaves on a bullet. Critics say a bullet lead match can mean little, while claims of a ballistics match often boil down to "I know it when I see it."
Jacqueline Behn, whose brother, Michael, is now in a New Jersey prison, says she's waiting with "bated breath" for a report on lead matching, due out any day now from the National Academy of Sciences. Six years ago, Michael was convicted of murder largely because a box of cartridges he owned had the same chemical fingerprint as bullets found at the murder scene. Experts have testified at hundreds of trials that bullets with matching lead profiles must have been made from the same batch of metal--and thus could have had the same purchaser.
But Behn enlisted outside scientists to look at the technique. They found that batches of lead actually aren't necessarily unique, raising the chance of a random match. These days, researchers like statistician Alicia Carriquiry of Iowa State University say that it's "reckless" to give much weight to a lead match. The technique may soon get an official no-confidence vote: One insider says the upcoming NAS report should put an end to "business as usual" for lead matching.
Ballistics evidence like that introduced in the sniper trial--based on the marks and grooves on bullets and casings--faces a different kind of challenge. Here, the question is: What makes a match?
Bar code. Spent cartridge cases bear impressions from gun parts like the firing pin, and bullets flying down a gun barrel pick up long scratches that look like a bar code. These marks can reveal the gun model, and experts say that the subtlest of them often amount to a "fingerprint" unique to a single gun. When examiners see "sufficient agreement" between cartridges or bullets from the crime scene and ones test-fired from a suspect's gun, they declare a match. For comparison, they also look at "known nonmatches," ammunition fired from other, similar guns.
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