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
Limitations of Ballistic Imaging
Ballistic imaging has obvious limitations that become more significant when we consider image databases containing all guns, or all new guns, rather than only criminal guns. Even when limited to new guns, the usefulness of an immense database as a crime fighting tool is questionable for a number of reasons.
"Wear caused by the friction of bullets traveling down a gun's barrel will change its 'signature.'"
Identical Marks. Initially, all guns of the same model from the same manufacturer will produce similar marks; guns produced by the very same equipment (perhaps only minutes apart) will be especially similar.19 This means that even the best search algorithm will develop relatively long lists of "possible" guns that need to be test fired so the bullets and cartridge cases can be microscopically compared to the evidence.
Worn Barrels. Over time, wear caused by the friction of bullets traveling down a gun's barrel will change the barrel's "signature," producing different ballistic images for bullets fired when the gun was new and those fired later.20
A barrel's ballistic signature often changes much more rapidly when the barrel is new; after use, the barrel stabilizes. Sometimes the five-thousandth bullet fired through a gun will match the first; at other times, consecutively fired bullets will not match.21 This is especially true for firearms that are used with high-powered magnum ammunition. Maryland and New York currently require the collection of ballistic images from new guns. The images from these guns may be significantly different from the images produced once the gun stabilizes.
Inexpensive guns, which are made from softer metals, wear more quickly. How often a gun is cleaned also affects the rate of change of the ballistic images.22
Replacement Parts. Replacing parts of the gun may change the ballistic image. A basic part of a gun is the receiver or frame from which the barrel, stock and other parts may be detached. Receivers have serial numbers and are generally regulated the same as complete firearms. However, many gun parts do not have identifying serial numbers, although replacing them changes the ballistic images the firearm produces:
- It is common, especially among shooting sports competitors, to replace a gun's barrel, firing pin or ejector.
- Gun barrels, triggers, grips and other replacement parts do not have serial numbers and are not regulated the same as complete firearms.
- To make a comprehensive ballistic registry work, all barrels, slides, extractors and firing pins would have to be serialized and regulated as if they were complete firearms.
Differences in Ammunition. Ammunition of the same caliber leaves significantly different markings:
- Firing different ammunition from different manufacturers may vary the marks from the same gun.23
- Using frangible ammunition, which shatters into many small pieces on impact, also defeats ballistic identification.
Reloaded Ammunition. Cartridge cases often are recycled. Empty cases "reloaded" with a new bullet and gunpowder are less expensive than new ammunition. Many target shooters save money by reloading their own ammunition from kits; other shooters purchase reloaded ammunition at stores or gun shows. Reloaded ammunition often ends up being fired through a number of different firearms. In such cases, purchasers of reloaded ammunition possess cartridges with markings from many different guns; these markings then combine with the markings from the purchaser's own guns as he or she fires the reloaded ammunition. Other Ways of Altering Ballistic Images. Ballistic markings can be varied in other ways:
- The markings on a barrel, ejector or firing pin can be changed with a steel brush, nail file or patch soaked in an abrasive.24
- The marking also can be changed by shooting ammunition with dirt, grit or grinding powder on it, or by polishing.
- Even putting toothpaste on a cartridge before firing may change its ballistic image.
"A gun's ballistic image can be altered repeatedly after crimes are committed."
Altered Serial Numbers. It is already common for criminals, especially black-market firearms dealers, to destroy the serial number of a gun. As ballistics databases are developed, it is likely that some criminals will change a gun's ballistic markings through one of the above methods - all of which are considerably easier and less obvious than removing a gun's serial numbers. A gun's ballistic image can be altered at leisure and altered repeatedly after crimes are committed. For example, it takes about five minutes to lightly file a gun's firing pin and breech face signature to make the cartridge unrecognizable by IBIS.25 Not all alterations succeed, of course, just as not all attempts to file off a serial number completely obliterate the number.26
Once fingerprinting was invented and popularized, many criminals adapted by wearing gloves - even though gloves reduce manual dexterity and may look conspicuous. Today, some rapists wear condoms to avoid DNA identification. In Boston, where BATFE and the police have aggressively sought to match crime scene cartridge cases with guns recovered from criminals, preliminary evidence suggests that some criminals have switched from semiautomatic pistols to revolvers, which do not leave cases.27
Examination of the King Assassination Gun. It is true that some criminals would be too careless to conceal ballistic images, but even when no efforts at concealment are made, ballistic images can change rapidly. For example, in 1997 the family of the late Dr. Martin Luther King Jr. filed a lawsuit over what it believed to be a cover-up of the circumstances of the 1968 assassination. At the direction of a court, a select group of forensics experts fired 18 rounds through the almost-unused Remington rifle the FBI said was the murder weapon. Not only did none of the 18 bullets from the rifle match the bullet that killed Dr. King, none of the bullets matched each other. "Every test bullet was different because it was going over [copper] plating created by the previous bullet," explained a retired Connecticut police forensic examiner who served on the team.28