Methods of comparison

Before the advent of the comparison microscope in firearms identifications in the 1920's, and for some time thereafter, bullet matchings (and shellmatchings also) were sometimes made by. other methods now seldom used. Sometimes a convincing identification of a bullet could be made by measuring in sequence the widths of the grooves on the evidence and test bullets and comparing them. These measurements were made with a filar micrometer (Fig. 13), an instrument readily obtainable. This is a special device placed at the top of a compound microscope. It has a scale and a cross hair which moves along this scale (or, as in the Spencer microscope, a scale which moves) as a calibrated drum is turned. One observes the bullet through the scale. The drum is rotated (clockwise, to avoid lost motion) until the cross hair is lined up with one edge of the groove and a reading of its position taken. Then a similar setting is made and reading taken on the opposite side of the groove. The difference is a measure of the width of the groove. The micrometer has to be calibrated before use, against an accurate scale placed on the stage of the microscope. Any change in the tube length (i.e., the distance between objective and eyepiece) destroys the calibration since it changes the magnification, so it is safest to recalibrate every time the micrometer is used. When properly used it is quite accurate. However, as will be discussed later, we now have better methods of measuring groove widths.

The width of each groove on the bullet is measured, proceeding clockwise, in sequence around the bullet, and the readings are then tabulated in order. If there is one groove on the evidence bullet and on the test bullet that is unusual in any way (as to width or marking) this may be called No. 1 for each bullet, and the readings then fall into order. One example of several identifications which were made by the author previous to 1925 is shown in Table 1.

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