Development of firing pin and breech face impression on lead and plasticine surfaces- a case report

Linking of the firearm, fired cartridge case and bullet is a classical example of tool mark identification. It follows the forensic science basic principle of individuality that no two tools marks should produce the same microscopic marks on two separate objects that they would be inaccurately or wrongly identified (Smith et al., 2016). Fired cartridge cases and bullets bear characteristic marks, which enable firearm examiners to identify the firearm that fired them (Kinder et al., 2004). Examination of firing pin and breech face impression on the fired cartridge case is essential link for the linkage of firearm with the fired cartridges (Sharma, 1963). But in few cases it becomes difficult to match the fired cartridge with the alleged firearm due to malfunctioning of firearm or in cases where the firearm misfires live test cartridges. In such cases the firing impressions could be reproduced on various soft surfaces without damaging the firing pin. Firing pin impressions are generated because of striking of firing pin on the percussion cap of the cartridge. Similarly breech face impression are obtained when a cartridge is fired in the gun the very heavy pressure of the gas generated inside the cartridge force the head of the cartridge back against the head of the breech block which imprints the impression on the breech on the brittle material commonly brass or germanium silver (Harding-Barlow, 1950).

In this paper, the breech and firing impression were reproduced on different surfaces to match the impression with the fired shell. First surface selected is Plasticine. Plasticine is a trademark name for an oil-based modeling material that was developed in England in 1897. Unlike clay and wax, plasticine stays soft and workable: it neither hardens nor dries (http://www.kirkside.com.au/Uploads/Images/plasticene.pdf). It has wide range of application in forensic science. It has been used successfully in examination of footwear impression (Morgan et al., 2009), facial reconstruction (Stephan & Henneberg, 2001; Benazzi et al., 2009; Claes et al., 2006; Cesarani et al., 2004; George, 1987), forensic ballistics (Jain et al., 2004; Hejna & Safr, 2009), restoration of obliterated marks (Kuppuswamy, 2011), tool mark impression (Rees, 1969) and Odontology (Gorea et al., 2007).

The second surface selected for the study is lead. It is heavy, soft, bluish metal, and occurs in nature in the form of ores. Being soft the marks could be easily reproduced on lead and it could be used as a surface for reproduction of marks. These findings are put into context for the comparison of cartridge cases.

Various casting methods (Plaster, Moulage, Faxfilm (Cowres, Davm L, Dodge, James K. A Method for Comparison of Tool Marks, 1948; Science and Practice Committee Report, 1949), plasticine (Kirk, 1953; O'hara & Osterbtrn, 1949), thermoplastics (Kirk, 1953), wax and electroplating (Tarko, 1948)) have been proposed for reproduction and comparison of individual characteristics marks on fired cartridge cases as an aid to firearms and tool mark identification. Most of the proposed methods had little or limited success due to time-consuming techniques and unsatisfactory results. In 1938, Moritz proposed a technique utilizing cellulose acetate with an acetone or amyl-acetate vehicle for the reproduction and comparison of the individual characteristics on cartridge cases, bullets and breech faces (Morrrz, 1938; Biasotti, 1956). The only advantage of the technique was preparation of flexible, translucent cast that could be flattened and viewed by transillumination thus reducing or obviating the difficulties inherent in the direct observation of curved or inaccessible surfaces by reflected illumination. Later, Faxfilm (trade name) became a commercial adaptation of the technique proposed by Moritz. It had larger application in studying the quality of finish on machined metal parts. The use of Faxfilm in law enforcement work has been reported, but literature on its practical application is meager (Science and Practice Committee Report, 1949; Biasotti, 1956). Cowles and Dodge have reported the successful application of Faxfilm to tool mark recovery and comparison and have pointed out its limitations when applied to deep impressions or to painted surfaces (Cowres, Davm L, Dodge, James K. A Method for Comparison of Tool Marks, 1948). Later another method for to form replicas of the fired bullets were proposed using sheets of vinyl thermoplastic varying in thickness from 0.030 to 0.125" (depending on the depth of the mark). The disadvantage of the method was the application of heat (100 to 125 °C.) and pressure (Kirk, 1953). An infrared lamp or electric hot plates were used for heating. These replicas were flattened and viewed by transillumination as either a positive or negative image. Accurate, reproducible replicas, free from air bubbles, shrinkage, and warping were obtained. However, the necessity of applying heat and pressure simultaneously, which may not be a serious disadvantage in the laboratory, makes this method rather impractical to use at the crime scene in the recovery of tool marks. Another material commonly used for lifting firing pin impression is plasticine, it is also used for replication of rifiling marks of fired bullet (Jain et al., 2004).

In the present study, plasticine and lead surfaces were used. The advantage of current method with previous methods reported is accurate, reproducible replicas, free from air bubbles and non application of heat and pressure for reproduction of marks. On comparison of firing pin and breech face impression of crime cartridge case with the impression of firing pin and breech face taken on plasticine. Three points in a match were found to be reproduced on the plasticine impression. Point-1 shows the periphery of the firing pin. Point-2 shows a horizontal line passing through the middle of firing pin impression. Point-3 shows a line at 7 O’Clock position (Fig. 4). However, the same impression was reproduced on lead surface was much pronounced than plasticine surface. Five match points were seen on the lead surface with clear demarcation (Fig. 5). Point-1 shows firing pin impression at 12 O’Clock position. Then moving down point-2 is a line at 2 O’Clock position. Point-3 shows an arc at 3 O’Clock position. In the middle a line crossing shows point-4. Point-5 is again a clear demarcation of firing pin impression at 6 O’Clock position.

On the comparison of marks on two substrates, lead was found most suitable surface for the reproduction of marks as lead surface gave more detail as compared to the plasticine. Further research could be carried out exploring different surface for the successive reproduction of the marks of firing pin and breech face in case of malfunctioning of firearm.