The quality and quantity of stria produced varies greatly and depends upon the pressure produced by the cartridge, the malleability of the cartridge case, the material the case has been manufactured from, whether the cartridge case has been plated, the depth of the depressions in the chamber and most important of all, the type of chamber depression.
Longitudinal fluted, scoop fluted and perforated chambers generally produce the best markings on the cartridge case, whilst helical and annular fluted chambers produce very little. With the PRC helically fluted chambers, the 7.62 x 17 mm ammunition is of very low pressure, and the lack of markings is probably more due to this than the depth of the helical flute itself.
Aluminium cases are better than brass at taking up any transfer of stria, and steel cases generally take none at all.
Any type of plating applied to the cartridge case will generally prevent any matchable stria being transferred, although with longitudinal fluting, the general outline of the markings will often be clearly visible.
Figure 4.21 Helical chamber marks on fired cartridge case.
Having said that, the longitudinal flutings found in most H&K weapons produce extremely clear stria on brass cartridge cases. These are normally readily matchable and can provide a valuable adjunct in the investigation of armed crime incidents.
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