FOHKNSIC MICROSCOPY 



weapon will be examined, proved in working 

 order, and will be fired into some collecting 

 medium, such as water, cotton batting or 

 waste, oiled sawdust, or anything soft 

 enough to stop the projectile without muti- 

 lating the surface. On the surface of the fired 

 bullet there will be parallel or slightly 

 diverging lines, inclined slightly from the 

 vertical axis of the bullet. The more promi- 

 nent visible grooves are the impressions of 

 the rifling of the barrel. These are the result of 

 a manufacturing operation which puts a very 

 slow-pitched series of grooves in the interior 

 of the barrel as shown in Figure 3; the pur- 

 pose of these is to cause the bullet to rotate 

 and produce gyroscopic stabihty in flight. 

 The raised portions in the barrel are referred 

 to as "lands" and the depressed areas as 

 "grooves". These, of course, produce their 

 negative counterpart on the fire projectile 

 so that a depressed area or groove on the 

 bullet corresponds to the land of the barrel. 

 The number of lands and grooves and their 

 inclination, as well as other dimensional 

 characteristics depend upon the manufac- 

 turer's notions as to what will produce the 

 most accurate and highest velocity projectile 

 when the gun is in use. Figure 4 shows fired 

 projectiles from two popular American hand- 

 guns. 



The identification procedure regarding 

 bullets involves first determining that all 

 class characteristics are in conformity. These 

 consist of the caliber of the weapon, the 

 number of lands and grooves and direction 

 of twists, the relative speed of twist, and the 

 width of the lands and grooves. Any major 

 deviation observable in class characteristics 

 immediately suggests nonidentity. The sec- 

 ond step in the identification or comparison 

 of the bullets is to place two tests from the 

 same weapon mider the objectives of the 

 comparison microscope and to study their 

 surface characteristics in order to determine 

 what sort of a "match picture" is presented 

 by tests fired from this particular weapon. 

 One of the test projectiles is then removed 

 and in its place, the evidence or "fatal" pro- 



jectile is substituted. A study is then made 

 of the surface of both bullets as viewed 

 through the comparison microscope, and by 

 suitable manipulations of their position the 

 technician attempts to match or line up the 

 individual characteristics until a pattern 

 closely dupUcating the comparison of the 

 two test bullets is achieved as shown in Fig- 

 ure 5. 



The basis upon which an identification is 

 made is not amenable to formula computa- 

 tion. Although both bullets might have been 

 fired in the same weapon, not all fine stria- 

 tions on one projectile will be found repro- 

 duced on the second projectile. The fine 

 striations observed will consist of those made 



Fig. 3. Cross section of .38 cal. Smith and 

 Wesson barrel showing land running at a slight 

 angle to the barrel axis. 



Fig. 4. Fired projectiles from .38 Special Colt 

 (left) and .38 Special Smith and Wesson (right) 

 revolvers. Land impression appears in the center 

 of each bullet. 



341 



