Ballistics (sometimes called “ballistic fingerprinting”) examines the markings that firearms leave on bullets to match a bullet to the firearm that fired it. Likening it to fingerprinting is an apt analogy. Like fingerprinting, ballistics suffers from a lack of independent verification of its assumptions.
Most firearms impart spin on a fired bullet, and subtle variations in the spirals within a firearm barrel leave tiny grooves called “striations” on the bullet. The striations a particular firearm imparts are supposedly unique. A technician attempts to match a bullet of unknown origin with the barrel through which the bullet was fired. However, it has not yet been proven that the striations are unique, but we know that striations are inconsistent. Different makers of bullets have different striations, even when they go through the same barrel. Wear and tear from regular use and intentional damage to the inside of the firearm barrel can also change striations.
Like fingerprinting, ballistics has to deal with the difference between lab conditions and real world conditions. Bullets fired in a laboratory are fired into a soft medium to keep the bullet intact, but bullets found at crime scenes are often damaged from smashing into hard materials. Ballistics labs do not have definitive guidelines for how much damage to a bullet is too much.
One expert may disagree with another about whether two bullets were fired from the same firearm. It is up to individual technicians to decide whether they feel comfortable that the striations on a bullet from a crime scene sufficiently match the striations from the suspected weapon. Ultimately, a human makes a judgment call, and no agency is conducting quality control to ensure the accuracy of a technician’s determination.