Cartridge Cases

Examination of a fired cartridge case may make possible the identification of a weapon in terms of type, make, and model. The presence of magazine markings, the type of breech-block mark, and the size, shape, and location of ejector and extractor marks are important imprints in making such identification. The size, shape, and location of the firing pin on fired rimfire cartridge cases can also be used to determine the make of the weapon. The appearance of the firing pin imprint from centerfire weapons may indicate the make of weapon used. Identification of a weapon as having fired a particular cartridge case can be made by comparing markings on test cartridges with those on the evidence cartridge. Again, it is extremely important that the same brand and preferably the same lot of ammunition be used for tests. In fact, this consistency is more important in cartridge case comparison than in bullet comparison.1

Figure 2.6 Markings on rifle cartridge case due to fluted chamber.

In rimfire cartridge cases, the firing pin impression is the most important identifying mark. Extractor, ejector, and breech block marks are less useful.1

Occasionally one will encounter a fired cartridge case having a series of parallel longitudinal markings impressed on the case (Figure 2.6).4 These marks may be either linear areas of swelling or linear deposits of soot. Such markings are a consequence of a fluted chamber. During manufacture, small parallel grooves have been cut into the wall of the chamber permitting powder gases to surround the cartridge case to allow the neck of the cartridge case to "float" on gas, thus aiding extraction. They are found in rifles, pistols, submachine guns, and machineguns. Flute marks may be present only on the neck or shoulder area of cases or along most of its length. The number of grooves may vary from 2 to 18. Heckler-Koch rifles, submachine guns, and pistols typically have fluted chambers. In the HK-4 pistol, there are 3 flutes that are designed to retard cartridge case extraction rather than facilitate it.

Some chambers have annular or helical grooves cut into the walls that retard extraction of cartridge cases by gas expanding the walls of the cases into the grooves. These weapons are relatively rare. Examples are the PRC Type 64 and 77 pistols.

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