Method of measurement

A number of instruments have been used by various investigators for the measurement of the widths of land and groove impressions on fired bullets, such as the filar micrometer, measuring microscope, traveling microscope (comparator ), tool maker's microscope, etc. (3, 4) (Fig. 84). All of these have been tried out in the author's laboratory, and reasonably satisfactory measurements can be made with any of them on bullets fired from new guns, where the edges of the land impression will usually be cleanly cut clear to the bottom of the impression (where the measurement should be made). But they are not entirely satisfactory for measurements made on bullets fired from guns that have seen considerable use and, in consequence, have lands whose edges are slightly rounded. The difficulty is of varying degrees, naturally, depending on the extent of wear on the lands and also on whether metaljacketed or plain lead bullets are being measured. Lead bullets cause more trouble than jacketed bullets. If the land impressions are not cleanly cut, it is, of course, difficult to see just where the cross hair (reticle) should be set to coincide with the bottom edge of the impression. This difficulty can be largely overcome by the use of a binocular microscope which gives a three-dimensional view of the impressions and thereby reveals details which are not visible (or are confused) when an ordinary microscope is used as the observing instrument.

A method developed by the author was used and the apparatus is shown in Fig. 85. This method involves the use of a Greenough-type binocular microscope (approximate magnification 20) with a fine cross hair (reticle-consisting of alternating narrow dashes and tiny circles) in one eyepiece and a measuring device (Fig. 86) mounted firmly on the stage. The bullet is caused to move to the right or the left by turning the micrometer spindle. Rotating it clockwise causes the bullet to move to the right (Fig. 85). When it is rotated counterclockwise a coil spring in the base of the device (which presses the bullet mount against the end of the spindle) causes the bullet to move to the left. This permits exact settings of the bullet such that the reticle will coincide with the bottom edge of the land impression.

By taking measurements at the bottom of the land impression one gets reproducible data, irrespective of the depth of the impression. Land impressions may vary in depth from bullet to bullet because of slight differences in their diameters. Naturally, the larger the diameter the deeper the impression will be. And if jacketed bullets are used the depth of the impressions will usually be shallower because of the greater hardness of the jacket. Whether the impression is deep or shallow, the width of the groove at the bottom will be the same if the impression is clean cut, barring other factors. Frequently the lands are narrower at the top than at the base and, consequently, the grooves produced by them will be wider at the top than at the bottom. This is another reason why the measurements should be made at the bottom of the groove. Occasionally the lands are rounded on the edges, either intentionally (rarely) or because of excessive use. In such cases the measurements will be more difficult and the values obtained less certain.

Several settings on each edge (driving and trailing edges) are made and these are averaged and recorded as the proper setting for that edge. The difference between the averaged right hand and left hand settings represents the width of the land impression.

It will be noted that only one eyepiece is provided with a cross hair. The final setting of the cross hair to coincide with the edge of the groove is made with this half of the optical system. The purpose of the binocular microscope is to furnish a three-dimensional view of the groove and thus enable the operator to study the exact shape of the groove so that he can determine the exact point at which the cross hair should be set.

Before any settings are made the bullet must be carefully positioned (rotated) until each edge of the land impression being measured is in perfect focus at the time the setting is made (otherwise the two edges will not be in the same horizontal plane) and the eyepiece is rotated to line up the reticle with the edges of the impression. Each land impression on the bullet is measured in this manner, and the average of all these measurements represents the average width of the lands in the gun from which the bullet was fired. If one needs to know the width of the grooves in the gun it can be calculated (approximately) from the measured land widths, diameter measurements, and the number of grooves.

When plain lead bullets (or even jacketed bullets) are fired from a revolver, slippage of the bullet frequently occurs when it strikes the rifled part of the barrel. This is because the bullet has already acquired considerable momentum and consequently resists the change in direction required by the twist of the rifling. Slippage does not ordinarily occur when bullets are fired from a pistol or automatic pistol, because here the bullet nose is practically in contact with the rifling at the moment of firing. When slippage does occur one must be very careful in selecting the markings on the bullet which truly represent the width of the land, since the over-all width of the land impression will be different at the two ends, being wider than the land at the nose end of the bullet (Fig. 87).

One side of the land impression (the driving edge) will usually show a clear, distinct edge upon which the reticle is lined up. Then the bullet is moved across the field until the reticle coincides with the other edge of the land impression at the last point of contact of the bullet with the rifling (i.e., at the base end of the bullet). One will then usually observe an impression of the edge of the land which runs toward the nose end of the bullet and which will be parallel with the other edge of the groove upon which the reticle had been set. This impression is often faint (though not necessarily so) and requires proper illumination to bring it out in proper perspective, but with the binocular microscope it can be located and used for a proper setting of the micrometer. Micrometer settings made on land impression edges that are not parallel have no value.

The advantage of using a truly binocular microscope over a microscope with a single optical system is most striking, as it permits one to see the true shape of the edge of the land impression on the bullet. This three-dimensional view is very important as it enables one to see exactly where the reticle should be set, provided proper illumination is used. A two-tube fluorescent lamp (Burton-see Fig. 85) gives very satisfactory illumination and is recommended over the customary small spot lights.

To facilitate what is at best a tedious operation of making a very great number of readings, whose accuracy depends so much on getting exactly the right illumination for the proper setting of the bullet with respect to the eyepiece reticle, the author has devised a simple, rotatable table upon which the microscope is placed. In front of this is the two-tube fluorescent lamp, with the tubes set nearly vertically. The table consists of two 8-inch circular plates of aluminum, each 1/4 inch thick, between which are ball bearings. The upper plate of the table rotates with a touch of the finger and is quickly turned and adjusted to the position where the light is at the proper angle to reveal clearly the shape of the edge of the groove on the bullet. One who has used this simple device will never return to the practice of trying to get proper illumination by moving the light source about.

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