Slight variations of breech face marks and firing pin impressions over 3070 consecutive firings evaluated by Evofinder®

Abstract

To investigate if and how the breech face mark and firing pin impression change over repeated firings, we used an automated ballistic identification system Evofinder® to study cartridge cases fired from five Chinese Norinco QSZ-92 9×19mm pistols. Each pistol fired 3070 consecutive shots. The first 20 cartridge cases were all introduced into the system. From the 21st until the 3070th shot, every ten cartridge cases were collected and one random sample of each group was introduced as the 30th, 40th, …, 3070th round. Thus, we introduced 325 specimens for each pistol and established a database of 1625 specimens. We used the first and every 100th specimen of each firearm to start correlations in the database and analyzed the results, especially the results of specimen 0001, 1000, 2000, 3000. Consequently, specimens fired closer in sequence to the specimen under examination had higher similarity scores. In boxplots, the boxes of known match specimens were much higher than boxes of known non match specimens. The Evofinder® ranked nearly all known match specimens in the first position of either firing pin impression list or breech face mark list. In conclusion, the two marks changed over 3070 consecutive shots and cartridge cases of nearer shot numbers were more similar. However, the variations of marks from the same firearm have not exceeded the differences between marks of different firearms and have not hindered the Evofinder® to make nearly 100% correct correlations.