The criminal defense attorney defending such prosecutions needs a fundamental understanding of the firearms examiner's role in presenting evidence concerning the identification of firearms and ammunition. This is especially true in cases where such identifications are the lynchpin of the prosecution or defense case. Understanding how bullets and cartridge cases can be identified as having come from a specific weapon depends on some knowledge of how firearms are manufactured, particularly pistols and rifle barrels.
First, a hole is bored through a cylindrical bar of steel of the desired diameter. That diameter determines the caliber of the weapon. Next, after the hole is bored, twisting grooves (or rifling) are created inside the barrel. This process causes a fired bullet to spin as it leaves the barrel, giving it more rotational velocity and, consequently, more stability with better accuracy. Regardless of the manufacturing process, each barrel acquires minute marks, called striations (or striae) through minor accidental occurrences in the rifling process. These striations are supposedly not the same for any two barrels and are the basis for the "individuality" of each rifle barrel. Any bullet fired through the barrel will be effected by the unique rifling of that particular weapon.
If a bullet (or bullet fragment) is obtained during the police investigation, often the police want to determine whether a particular weapon fired it. Assuming the police also possess the suspected weapon, the process of identification is straight forward. First, the examiner fires a series of test bullets from the suspected weapon and uses them to compare with the "unknown" bullet obtained during the investigation. The examiner uses a binocular comparison microscope, which is an instrument consisting of two separate microscopes mounted side-by-side. The unknown bullet is placed under one microscope and the test bullet under the other. The examiner then scrutinizes both bullets and attempts to locate similar striations that "match" both the test and unknown bullets. In theory, a careful study of all the detail on both bullets permits the examiner to conclude whether both bullets were, or were not, fired through the same barrel.
In a later post, we'll discuss the identification of a cartridge case and whether is was fired from a particular weapon. Although different from bullet identification, the same principles of probability apply.