Other instruments used in forensic firearms examination

Apart from the everyday instruments commonly found in any forensic laboratory, for example, microscopes, micrometers, and so on, there are a number of others which, although probably no longer in common use, one should be aware of.

Rifling meter. In this instrument, the barrel of the weapon under examination is clamped to the bed of a lathe. Into the tailstock of the lathe is fixed a long steel rod, one end of which has a lead plug of the same diameter as the bore of the weapon. The other end of the rod has a graduated disc which rotates with the rod. As the rod is pushed down the bore of the weapon, the rifling bites into the plug on the end and causes the rod to turn. The degree of rotation of the disc is measured against the distance the rod travels down the bore.

There being very little necessity, in forensic firearms examination, to measure the actual rifling twist of a barrel, this instrument is hardly, if ever, encountered.

Comparison camera. Basically, a comparison camera is a plate camera with an exceedingly long body and two lenses. In front of the lenses are two bullet stages with operating rods stretching back to behind the plate holder. The operator sits behind the ground glass screen operating the bullet holders by remote control until a match is obtained. A plate is inserted and exposed in the normal way. This instrument is exceedingly difficult to operate and, as it has not been commonly used for over 35 years, it will probably only be encountered in a museum.

Tallysurf. In this instrument, a fine diamond-tipped stylus is drawn across the toolmark. The small variations in height caused by the striations are magnified and plotted on graph paper. These can then be compared without the aid of a comparison microscope. As toolmarks are never consistent along their total length, several passes have to be made before a representative sample can be obtained.

This instrument is possibly of some use in the comparison of toolmarks, but the stylus does permanently damage the toolmark. It also requires a great deal of experience to interpret the graphs produced and, even when a possible match is encountered, they nearly always have to be verified with a comparison microscope.

Tallyron. The tallyron has the same basic principle as a tallysurf, but it is designed for the examination of bullets. In this instrument, the stylus is stationary and the bullet rotates, giving a circular graph. It is of very little use other than as a rough screen for possible matches (Figure 4.3).

As with the tallysurf, the bullet is permanently damaged by the stylus. It also has the disadvantage that it is all but useless with even slightly damaged or distorted bullets.

It was, however, used to great effect in Northern Ireland to screen the vast number of bullets received for possible matches. These were then examined manually on a conventional comparison microscope. It is probably not used any more.

Peripheral camera. With this camera, a magnified image of the circumference of the bullet is obtained on a strip of film. As the bullet is rotated, the film passes by the bullet being exposed at the same time. Thus, all the grooves and lands are recorded on one continuous strip of film.

Figure 4.3 Print of tallyron (six-groove bullet).

Assuming that the lighting is correct, the bullet is not damaged and the magnification is sufficient to record the finest of stria, this instrument can be of some use.

It is, without prior knowledge of how the grooves match, necessary to photograph the entire surface of the bullet twice on one strip of film that all the lands and grooves may be compared on one continuous strip.

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  • Angelica
    Is there a single instrument use in forensoc ballistics?
    3 years ago
  • dawit girmay
    Is there a single instrument which can be used in forensic ballisticsWhy?
    3 years ago

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