Ballistic Imaging: Not Ready for Prime Time
Wednesday, April 30, 2003
by David B. Kopel, J.D., & H. Sterling Burnett, Ph.D.
Table of Contents
What Is Ballistic Imaging?
Ballistic imaging captures and stores a digital photograph of the striae (fine stripes) and other tiny marks on bullets and cartridge cases. Some gun control advocates refer to ballistic imaging as "ballistic fingerprinting," but this is misleading. Fingerprints are immutable. Ballistic markings can be made by several parts of a gun; they change over the life of a gun and can be easily altered by the gun owner.
"Only bullets used in rifles and handguns produce useful ballistic images."
Barrel Rifling. One way ballistic markings are created is by the spiraling grooves, or rifling, cut into the inside of the barrel of most rifles and handguns to make the bullet spin. (Spinning stabilizes the bullet in flight so that it travels a straighter path.) [See Figure I.] The grooves and the raised surfaces between them, called "lands," leave striae on the bullet. Microscopic imperfections in the gun barrel increase with wear and leave marks on the bullet.1 Of course, if a bullet deforms or is otherwise damaged when it hits a target, which often occurs, the ballistic images are much more difficult to read.2
Only bullets used in rifles and handguns produce useful ballistic images. Shotguns fire many small pellets at once. These pellets do not expand to the diameter of the barrel and therefore are not marked by it.
Marks Left by Other Parts. An unfired cartridge consists of a cartridge case that contains the bullet, the gunpowder and a primer. When a shooter pulls the trigger, the firing pin strikes the primer, starting the reaction that ignites the gunpowder. The expanding gases created by the burning gunpowder propel the bullet out of the cartridge case and down the barrel. When the cartridge has been fired, marks are left on the now-empty cartridge case and its spent primer by the firing pin, the bolt breech face, and the interior of the firing chamber. In semiautomatic guns, marks also are left by the extractor that assists in removing the expended case from the firing chamber and by the ejector that expels the case from the gun.3
Usually, no ejector markings remain on guns that require the shooter to manually remove the cartridge case; these include revolvers and derringers, single-shot firearms, and double-barreled rifles and shotguns.4 Semiautomatic guns automatically expel used cartridge cases, which criminals often leave behind. They are much less likely to leave their cartridge cases at the crime scene if they have to remove them manually.5
"A ballistic examiner must make the final judgment regarding both bullets and cartridges"
Computer Matching and the Ballistic Examiner. A ballistic examiner must make the final judgment regarding both bullets and cartridge cases.6 His determination is subjective, based on his professional experience - in contrast to DNA matching, which is objective.7 Computer ballistic image matches are not currently accepted as evidence in court without a final judgment from a ballistic examiner.8 The manual microscopic examination of several possible matches can take several hours.
Bullets and Cartridge Casings. In contrast to bullets, cartridge cases do not strike a target and thus are not deformed by impact. Case imaging is much better than bullet imaging at achieving "cold hits" - linking two items not already suspected to be linked.9 Cartridge case images change more slowly than do bullet images as a result of normal gun use. Accordingly, preliminary computer matches are easier to perform. Still, a ballistic expert must examine the image and make a subjective judgment.
Guns made by the Glock Company and others produce quite distinctive impressions on cartridge cases.10 Other guns produce useless casing images. For example, the markings on the cases of the tiny rimfire cartridges used in .22 and .25 caliber guns are much less forensically useful.11