One modern manufacturing process for producing a centerfire case starts with brass rod or wire, in coils. A machine called a cold header, similar to the one used to make common nails, feeds in the rod or wire, cuts off a piece large enough to make one case, and transfers it to a cavity in the machine, where it is struck by a punch. This process forms the irregularly shaped cylindrical piece into a precise sort of button shape. The button is annealed (heated and then cooled) to reduce its hardness, and is then fed into a two-stage transfer press that transforms the cartridge blank into a low, wide cup. The half-formed cup is next pushed through a die or series of dies that draw the blank to its final shape and dimensions. Additional annealing, cleaning, and forming steps are done sequentially until the blank is in the final shape of the cartridge case.
The last major component of the cartridge is the bullet or projectile. Bullets in modern ammunition can consist of a variety of metals. There are bullets made entirely of aluminum, steel, and sometimes brass; nonmetallic substances like rubber and wood have also been used to make bullets. However, to provide the needed weight for improved accuracy and performance, bullets most often contain some amount of lead.
Bullets are designed for two basic purposes—penetration on impact with a target and perforation and expansion to increase damage—and the exact composition and construction of bullets are tailored to those purposes. An all-lead bullet is very soft and therefore expands rapidly on striking a target. Indeed, “pure lead is not used for lead bullets” precisely because “it is too soft [and] damages too easily in handling and loading”; antimony is most commonly added to lead as a hardening agent, though tin has also been used (Frost, 1990:27). Better penetration power at greater distances and accuracy can be attained by covering a lead core with a full jacket or partial jacket composed of a copper alloy. High-velocity, fully jacketed bullets are designed to penetrate deeply, while lower velocity jacketed bullets may tumble within the target and cause additional damage due to expansion. Mushrooming or expanding bullets, such as hollowpoints, are designed to transfer a maximum amount of energy to the target and to penetrate but not exit. The composition and design of bullets—along with what materials they do or do not strike—are important to forensic ballistics analysis as they affect what condition a recovered bullet will be in and hence how difficult it is to match to other evidence.
A lubricant is applied to bullets before they are seated in cartridge casings; it acts to cut down on metal fouling of the bore, the deposition of particles or residues from the bullet (Frost, 1991:31). In centerfire cartridges, where “grease grooves” are created in the case by knurling, the