Let’s start with a subject that anyone who has watched Law & Order is familiar with: the use of bullets and cartridges to uniquely identify an individual firearm. For anyone who is unfamiliar with guns there are essentially three types: handguns, long guns or rifles, and shotguns. Handguns and rifles are rifled, that is they have curved grooves on the inside of the barrel to start the bullet spinning. The spaces between the cut grooves are called lands. They also use ammunition consisting of a bullet, case, primer, and propellant. Ammunition is classified according to how many thousands of an inch across they are (e.g. .22, .45, .357 MAGNUM THE MOST POWERFUL HANDGUN IN THE WORLD) or millimeters (e.g. 7.62). Rifles and pistols also come in a wide variety of firing types which, for the purposes of this discussion, break down into those which eject their cases upon firing, and those which retain bullet cases until manually ejected.
Shotguns are not rifled and use shotgun shells which consist of (most commonly) a case, propellant, primer, projectiles and wadding. Wadding keeps the pellets and powder in the right position in the shell. Shotguns can fire both shot, an amount of round projectiles, or a single large bullet, a slug. Their ammunition size is measured by gauge. Gauge is classified as the number of spherical balls of pure lead exactly the size of the bore that equals a pound.
After that introduction to guns, we can now discuss how firearms evidence is analyzed and classified. Experts break down this evidence into three categories of characteristics: (1) class; (2) subclass; and (3) individual. Class characteristics are those which are common to all firearms of that type. These tend to deal with rifling and caliber specifications. (e.g. direction of the rifling, diameter of lands and grooves, width of lands and grooves, number of grooves, degree of twist, bullet size)
Subclass characteristics are, allegedly, those which are imparted upon a firearm in a specific production line. For example a certain short run of a firearm might have a unique flaw in the barrel production, or have used an experimental alloy that changes similarly over time. Here we are getting into “made up science” territory. There is no standard for distinguishing subclass characteristics, or if they even really exist. There is no real difference as far as I can tell between subclass characteristics and individual characteristics. The reason you would say it’s subclass is because you can’t say its individual which means you are trying to link the suspect with something you are SURE is true but just can’t really prove beyond a hunch.
Individual characteristics are those which everyone has probably seen on TV. When a gun is fired the bullet and case obtain unique marks through that process. The individual manufacturing flaws in each handgun produce different and unique marks on a bullet. Experts claim that two bullets fired from two handguns produced from the same machinery one after the other can be uniquely distinguished. It is this assertion that is the basis of its admissibility.
In a firearms testing lab you create a test bullet through firing and then, under a microscope, compare the test bullet to the evidence bullet. If the two bullets were fired from the same gun then the striations should be a precise and unique match. That’s right ladies and gentlemen the process is simply a matter of lining up the edges like a jigsaw. It’s even simpler than you imagine because you can use a special microscope with a split down the middle of the viewing area allowing you to individually twist and turn each bullet until you prove or disprove a match. The hardest part of doing the comparison is learning to use such fine movements in the lining up and rotating of each bullet.
The marks on the bullet are created by unique characteristics in the barrel. As a firearm is used it creates more and more flaws and imperfections in the barrel making it easier and easier to uniquely identify a bullet fired from that gun. Physical imperfections from metal on metal action as well as chemical and heat based changes all impact the striations. Simple rust and time will create more uniqueness.
Before I go any further let me outline a key checklist of all forensic science. Is the “Thing” unique? Can it transfer that uniqueness to the evidence? Can that uniqueness be sufficiently analyzed? To what level of accuracy and precision? These are questions which must be answered in every single area of forensics before they can be considered “good” evidence. You will now see why many aspects of forensic science are not “good.”
Remember above where I pointed out that experts claim they can tell the difference between two bullets fired from two guns which were produced one after the other on the same line? I don’t think they are lying but I do think they potentially seeing things which aren’t there. Almost no forensic studies are blinded. Those that are blinded tend to be ended early and have the conclusions and results destroyed. From what I have read of firearms studies I have no indication they are blinded. That is, an examiner will be told they are two bullets from two different handguns. The examiner will, knowing they are different, find the differences, because to do otherwise would be to admit either (1) that the science is bad or (2) that they are a bad examiner. Neither of these are acceptable conclusions.
Firearms ID is done entirely based on a gut feeling of examiners. There have been no statistical studies on bullet uniqueness of any sort. There have been multiple studies which concluded that there is no objective criterion for analysis of bullets and that there aren’t enough statistics to conclusively say how unique the impressions left on fired bullets are.
Furthermore, even if striation patterns on the bullets are the same, there is presently no reason to conclude, beyond the personal experience of examiners, that you can decide they are from the same gun. An expert can testify for hours as to all the unique identifiers which make the bullets the same but they have no basis to conclude that this means the bullets were fired from the same firearm. This goes back to the inability to effectively distinguish between class, subclass and individual characteristics. In a proficiency study in 1978 the error rate was almost 30%. More recent tests showed far more accuracy but it’s impossible to know whether or not that means proficiency has increased or (more likely) the studies were poorly designed and made to ensure low error rates. In fact there is some evidence that later tests were so elementary and so assisted that they are effectively meaningless.
Examiners can also look at cartridge cases with largely the same analysis. The same criticisms apply. One major difference is that cartridges receive a sudden sharp hit and are then ejected or retained. Those that are ejected receive additional marks from the process of ejection. Bullets receive a long sliding and grinding action. From experience, I can tell you that cases are a hell of a lot easier to match up than bullets. Experts generally claim the same.
In case you are wondering courts have heard all of this and are becoming VERY skeptical. While this kind of evidence is still almost always admissible (with proper support of course) it is surrounded with caveats, special explanations and the words “consistent with” and “could” or “likely” instead of what used to be a “match.”
Edit: Pretend I also discussed that guns change over use so that a bullet fired from when a gun was first produced wouldn’t necessarily match a bullet fired from the same gun after 20 or 30 rounds.
Edit2: Pretend I also brought up that bullets fired “in the field” so to speak often come up extremely deformed or outright obliterated so oftentimes there might not even be a sample to review!