Crime Library: Criminal Minds and Methods

The Mistress of Hollywood: June Cassandra Mincher

Firearms Examination

Matching an ejected casing to a gun means shooting the suspect gun (if recovered) in the lab's firing range.  Then a comparison can be done between the shell casing from the scene and the one the examiner shot.  People sometimes mix different brands of ammunition, so it's necessary to use the brand under investigation.  Since the test bullet must be recovered, the gun is fired either into a tank of water for very soft metals or into thick cotton batting for others.  Then it's compared for similarity of microscopic scratches.  At least two bullets should be fired for test purposes, says Robertson, to ensure that the firearm is producing individual characteristics.  They're compared in the same way that a test-fired casing is compared to the evidence casing.

On a comparison microscope, views of the casings are linked optically.  It takes skill and experience to make a definitive match, but it's possible to say that a certain bullet came from a certain gun, and only that gun. 

Edward Robinson
Edward Robinson

"A firearm in the area of the breach," Robertson said, "is generally made of steel and the cartridge case is made out of brass.  So the chamber portion of the barrel, the firing pin, extractor, or any metal part that comes in contact with that cartridge case can lead to the tool mark that can be identified back to the specific area of the firearm."  He would then set the casings on a comparison microscope and do an analysis.  If the shells from the scene are marked in the same area as the test-fired shells, then he could advance further to look at finer indicators on the shells that make the tool marks distinct and individual.  "It's a matter of pattern recognition."

Criminalist Richard Marouka
Criminalist Richard Marouka

Criminalist Richard Marouka with the LAPD Firearms Identification Division adds that bullets are weighed as well, to help determine their physical characteristics.  "The lands and grooves can be optically measured and those measurements can be cross-referenced to a book we maintain in our laboratory that will denote specific types of firearms that could have fired those bullets.  No two fired cartridge cases from two different firearms will be exactly the same."

In 1984, he says, there were no digitalized databases, as there are today, for computerized comparisons.  It fell to an experienced examiner to make the identification. "A qualified analyst," says Marouka, "has the ability to say with one hundred percent certainty that if a fired cartridge case was fired in a specific firearm, based upon its individual markings, it was."

If the gun is not recovered, there's another approach.

 

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