Ballistic Fingerprinting of Guns: An expensive failure

Since 2001, all pistols and revolvers in New York state have been ballistically fingerprinted before sale, and this information has been recorded in a database.  The (faulty) idea was that this database, which costs $1 Million each year, would help the police to solve gun related crimes.  However there have been exactly 0 prosecutions that can be attributed to this database.

The (faulty) theory behind ballistic fingerprinting
Proponents of ballistic fingerprinting believe that having the police fire a gun (and keep a shell casing) before that gun is purchased will allow the police to determine whether that gun was later used at a crime scene.  To make this determination, they say, the markings on the shell casing that the police retained from before the gun was purchased would be compared to the markings on the shell casing found at the crime scene.  Proponents also believe that such fingerprinting can be done with bullets.

Why ballistic fingerprinting doesn’t work
Ballistic fingerprinting, as NY has tried, sounds like a good idea in theory – but it simply does not work in practice for the reasons discussed below:
1. Criminals don’t buy their guns legally – Criminals don’t buy their guns legally, which means the police don’t have a ballistic fingerprint on file for illegal guns.  Instead, criminals steal guns or buy them on the black market, which bypasses ballistic fingerprinting, waiting periods, etc.
2. The markings a gun leaves on casings can change – A gun leaves marks on the shell casings it uses when its ejector and other internal components touch the casing.  However the markings that each gun leaves can change over time.  First, normal wear and tear on the gun will wear down the ejector and other parts that mark the casings, causing the marks to be different from those left on the first few shells that it fires.  Secondly, replacing the ejector or other parts of the gun will change the markings.  Replacing these parts could be a part of normal and legitimate repair work, or an intentional action by a criminal to change their gun’s “fingerprint”.  Regardless, the result is the same.
3. The markings a gun leaves on bullets that it fires can change – A gun leaves marks on bullets it fires because of the rifling (grooves that make the bullet spin for greater accuracy) in the barrel.  These groves wear down over time, and can be intentionally changed by a person with a tube of toothpaste and sandpaper.  Replacing the barrel of the gun also changes the markings on the bullet.

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Ballistic fingerprinting simply does not work, as the state of New York has learned.  Criminals don’t buy their guns from lawful gun dealers, which means that no ballistic fingerprint is tied to the criminal.  Furthermore, normal use of a gun causes its ballistic fingerprint to change – and a criminal can take simple steps to drastically change the ballistic fingerprint.