Bradford and Brackett (3) have pointed out that when the distance between the two edges of a groove on a bullet is measured by the optical method, one measures a distance on a plane surface and not actually the curved distance (following the contour of the bullet) which they consider to be the true width of the groove. They give a formula whereby the difference can be calculated and a series of curves showing the corrections for various groove widths on bullets from .22 to .45 cal. In the case of the .38 cal. bullets used in this investigation, having a land width of about 0.10 inch, the correction amounts to +0.0014 inch. Surely no fault can be found with their statements-but, actually, in the practice of gun identification it does not matter, because if one uses the same system of measurement on the evidence bullet and on the test bullet he gets his answer as to identity or nonidentity in either case, if a positive identification by such measurements is possible. As a matter of fact, this method is rarely used nowadays to determine identity of a particular gun because the comparison microscope (or, better, the comparison camera) enables one to establish identity or nonidentity more quickly and more strikingly, and photographs of the matches between the evidence and the test bullets can be made and shown to the jury.
A more useful application of a knowledge of groove widths on fired bullets is to assist in the identification of the make (and, sometimes, the model) of gun that fired an evidence bullet when no suspect gun is yet at hand. If one knows the number and widths of the lands in the gun, the direction and pitch of the rifling twist, and the caliber (actual, not nominal)-all of which can be obtained from the evidence bullet if it is in good condition-he is in a position to narrow down the search for the weapon very materially. If his information is sufficiently complete, he may be able to narrow the search down to a single make or at least to two, three, or four makes. This is possible only if one has extensive information concerning rifling characteristics as they actually exist in guns, rather than in manufacturers' tables. The „book values" and the actual values are often quite different-and for many guns even book values are not obtainable. Like typewriter fonts, rifling characteristics are subject to intentional change from time to time as well as to the unintentional errors made during the process of manufacture. Unfortunately, in the case of many of the cheaper foreign guns and particularly those made years ago, little if any attention was paid to the matter of rifling „specifications." This state of affairs is unfortunate for the firearms examiner as it naturally makes his task of identification of make and model of gun more difficult, and sometimes impossible. One American manufacturer (of a very popular gun) very frankly states that his company does not consider that any specification other than bore diameter is of any particular importance in the proper functioning of their guns. It is fortunate for the firearms examiner that all manufacturers are not of this opinion!
The manufacturer's specifications call for a land width of 0.1034 inch for the particular model of S&W revolvers used to fire the bullets whose measurements appear in Tables 4 and 5, but in the many measurements made in the author's laboratory no width as great as this has ever been found for this model of revolver-new or used. It is clear, however, that the lands become narrower as the gun is used. Of course, the edges become worn and this makes a micrometer setting more difficult as the imprint of the land is not as clean cut. The difficulty is greatly reduced, however, by the use of the binocular microscope which reveals the true shape of the groove, when proper lighting is used. This method has been found to be superior to the filar micrometer, the tool maker's microscope, the comparator, or the traveling-stage microscope, in all of which a single eyepiece is used in the observing instrument. A three-dimensional view is necessary to see the true shape of the groove edge.
The measurement of land impressions is a necessary procedure in establishing the rifling characteristics present in a particular arm. The actual widths of the lands may be determined very accurately by measuring the widths of the impressions made by the lands on either a bullet or a lead disk which has been pushed through a barrel under moderate pressure (Fig. 88). In either case the measurements are made with the equipment described in the first part of this section and shown in Fig. 85.
Frequently it is found advantageous to push the disk into the barrel for only a short distance (onehalf inch or so) and then remove it for measurement. Clearer impressions are often so obtained. The author makes it a practice to measure all of the land impressions present and to average them. The data recorded in the tables given in a later chapter were so obtained.
In a paper relating to this subject (4), B. D. Munhall concludes that groove widths cannot be measured with a degree of accuracy greater than 0.01 inch. But it would seem from studies made in the author's laboratory that a higher order of accuracy is possible with the apparatus and techniques here described. The reproducibility of the average widths of land impressions on bullets fired from the same gun, as so measured, seems to be of the order of ±0.001 inch when proper ammunition is used, even in guns that have seen much service. As is well recognized, it is abuse rather than use that is most harmful to the rifling in a gun.
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