FIREARMS EXAMINER
The first and most basic comparison for an examiner is to make sure that the ammunition is exactly the same. In other words, if the examiner is comparing two bullets, they must be of the same caliber and brand. This sounds too simplistic, but often these bullets are mutilated from going through a body, or a wall or other barrier.
Firearms Examiners compare bullets, as well as cartridge cases by the marks that are produced when the weapon was fired. In every firearm, there are long scrape marks dug out of the inside of the barrel when produced, to give the bullet a spin as it shot out. This is called rifling. The spin that it takes ensures more accuracy of the shot. As the bullet goes down that barrel, the scraping makes marks on the bullet, called lands and grooves. The common knowledge has been that each gun has these marks are slightly different in every gun. The firearms examiner looks carefully through a microscope to find and identify every single little nick and imperfection to compare a “found” bullet to the “known” bullet, which possibly came from the victim of a shooting.
Along with this rifling, there are other impressions that might show up, due to a firing pin, ejector marks, or other processes interior to the firearm which may show up. Ejector marks might be made on a cartridge case when it is pushed out of the chamber by the mechanics of the firearm. These can also be compared to each other by the firearms examiner.
If the question at hand is whether or not a found bullet or cartridge has been fired from a particular gun, the examiner fires a bullet into a tank of water and then compares the resulting bullet to the one they had. Other tests include firing at a regular firing range if there is a question of trajectory.
One of the defenses to challenge the results of a firearm examiner’s report is that the manufacture of firearms today, is highly automated and of a much more high-tech material than in years past. Therefore, each firearm coming off the assembly line will be virtually identically to every other one. And the material would not deteriorate into its own distinctive rifling very easily. The lands and grooves of a bullet coming out of any of these firearms, would therefore, be difficult to distinguish.
Another argument would be that with the mass market sale of guns and rifles in stores like WalMart, the entire inventory of them probably came from the same factory, the same production line, and the same batch of materials. Another reason the defense could say that the close comparison of lands and groove marks do not mean a “match.”