FORENSIC MICROSCOPY 



Fig. 5. Comparison photomicrograph of two projectiles fired from the same weapon. 



by fixed structures in the interior of the 

 barrel and those produced by transient ma- 

 terial such as rust, unburned powder, ac- 

 cumulated carbon and metallic deposits, 

 and so forth. Consequently, it is necessary 

 for the examiner to study thoroughly the 

 pattern presented by two tests to differenti- 

 ate between striations produced by signifi- 

 cant, stable and permanent imperfections 

 and those of a transient nature. Because of 

 the complexity presented by this changing 

 pattern, it is not possible to establish any 

 percentage criteria of matching lines by 

 which an identification can be assured. Only 

 through the experience of having studied the 

 comparison of numerous projectiles can a 

 technician arrive at a properly qualified 

 opinion as to the identity of the source of 

 two projectiles. After a "match" has been 

 determined, it is possible to take photographs 

 through the comparison microscope. As a 

 rule, this final procedure is not employed 

 smce the presentation of photographs in 

 court is not often helpful to the jury in mak- 

 ing a decision in the case. 



Of equal importance in the field of firearms 

 identification are the fired cartridge cases 

 ejected at the scene from either hand-oper- 

 ated or semi-automatic or automatic weap- 

 ons. Wlien a cartridge is ignited in a weapon, 

 the high pressures developed force the rear 



face or head or the cartridge case against a 

 vertical surface, called the breech-face. At 

 the time of this action, any imperfections, 

 striations, pits, and so forth on the surface of 

 the breech-face are recorded as negative im- 

 pressions on the primer of the cartridge case. 

 These breech block markings, in conjunction 

 with the firing pin impression, represent val- 

 uable areas for identity study. Their useful- 

 ness and significance is on a par with rifling 

 impression on the fired projectile. In addition 

 to these two areas, an automatic weapon im- 

 parts additional identification features in the 

 form of extractor and ejector marks resulting 

 from the cyclic operation of the weapon. All 

 these features, separately or together, can af- 

 ford sufficient individual characteristics to 

 substantiate an identification of two car- 

 tridge cases as having a common origin. In a 

 broad sense, the same philosophy and pro- 

 cedure is followed as would be followed in an 

 identification of a fired projectile. Figures 6 

 and 7 show examples of identification 

 through breech block markings and ejector 

 markings. 



The results of firearms identification and 

 comparative micrography examinations are 

 expressed in court as opinions. As indicated, 

 it is not possible to subject the observations 

 made to the rigorous procedures of mathe- 

 matics. Therefore, the opinion rendered is 



342 



