the machining operations of reaming the bore and rifling the grooves. Any such machining operation will leave the bore at least slightly rough, and each rough spot will leave a mark on the bullet during its passage through the bore.”
The rifling carved into the barrel takes the form of grooves separated by raised areas, known as lands. These lands and grooves create corresponding engraved areas—dubbed land engraved areas and groove engraved areas (and commonly abbreviated as LEAs and GEAs)—on the bullet surface, separated by shoulders. The land engraved areas, being the part of the bullets that scrape against the raised lands on the barrel, are the principal areas of interest for observing striations.
The pattern of land and groove engraved areas on recovered bullets can be used to determine basic information about the rifling characteristics of the gun that fired them, in order to identify a class of guns from which it came. Specifically, the number of lands is an important class characteristic, as is the direction of twist evident from a side view of the bullet. Bullets (and corresponding rifling characteristics) are commonly labeled by these two pieces of information—e.g., 5R for five lands and a right-hand twist. A recovered bullet can also be measured to suggest the caliber of the ammunition and weapon. However, this is not always possible—nor is a full analysis of striation marks—due to the condition of some bullets recovered from crime scenes (and victims).
Bullets fired through weapons using polygonal rifling create special difficulties. Compared to conventional, square-edged rifling, polygonal rifling has key advantages: it reduces metal fouling, and it increases bullet velocity by reducing friction as the bullet passes through the barrel (Heard, 1997:123). However, the smoothness and subtlety of polygonal rifling can make it difficult to discern even gross features on recovered bullets—the shoulders defining lands and grooves—much less fine individual detail. Heard (1997:131) concludes that “generally speaking it is possible, although extremely difficult, to match bullets from polygonally rifled barrels.”
The underlying theory of firearms identification depends critically on manufacturing processes, positing that the tools used to form component parts wear with use so that each part may share the same gross features yet differ in microscopic (and, presumably, uniquely individual) ways. Manufacturing processes are also essential to consider in assessing the costs and benefits of wide-scale ballistic imaging or alternatives such as microstamping. Introducing stages to the process of producing firearms or ammunition—for example, systematic test-firing to produce exhibit cases, imaging of exhibits in large batches, or laser-etching a unique mark on the