Some firearms examiners resort to a visual comparison of the slopes of the rifling grooves of the evidence bullet with the slopes on bullets which have been fired from guns whose rifling twist is known. This comparison can be done with the bullets placed under the comparison microscope or, still better, with the comparison camera where the enlarged images of the bullets are viewed on a ground glass.
In either case the two bullets are placed in the usual positions and well-defined grooves are brought into juxtaposition, care being taken to see that the sides of the two bullets are exactly parallel. If, under these conditions, the slopes of the grooves on the two bullets appear to be exactly the same and if the widths of the grooves are alike, the probability that the two bullets were fired from guns having the same rifling characteristics is great.
Obviously this method, in common with the other methods described, can give useful information only in cases where the evidence bullet is in good condition. For deformed bullets no method will work. For the practical use of the method the examiner should have at hand a considerable collection of bullets which have been fired from guns having known rifting twists, and, of course, these bullets also must be in good condition.
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