Firearms Identification the Laboratory Examination by J. H. Mathews

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An unusual case of „firing pin drag,“ caused by unlocking of the tip-up barrel of a shotgun at moment of explosion. Fig. 41. Left: Shell head fired in Sauer Auto Pistol showing cone-shaped mark on primer. Right: Breechblock in Sauer Auto Pistol showing cause of mark. Fig. 42. Marks made by shell striking slide. In the ejection of shells from some makes of automatic pistols the shell often (if not invariably) strikes the edge of the port in the slide and a distinctive mark often results. In some cases these can be matched, in others they cannot.

Left: Test shell from Sacco's automatic. Right: Evidence shell. Comparison microscope was not used in the investigations. Similarities in the breechblock markings are obvious, however. Fig. 27. A rather unusual number of markings found on each primer of shells fired in this gun. Fig. 28. Breechblock markings. Left: Test shell fired from suspect's gun. Right: Fatal shell found at murder scene. Fig. 29. Primers on evidence shell (left) and test shell (right) show same Breechblock marks and same firing pin drag.

No comparison microscope photographs of matchings were introduced, however; only enlarged photographs of the head of the shells. When these were examined (during cross-examination, at the request of the State's Attorney) it was discovered that in photographing one shell head the illumination had come from the right side while in the other the illumination had come from the left side! Since experts on both sides had testified that the illumination must always come from the same side and from the same angle, cross-examination along this line ceased abruptly!