While the following method does not have the precision of the method already described it has the advantage that the equipment necessary is more likely to be available. It requires a good compound microscope provided with a well-made mechanical stage which includes a graduated circle and a vernier scale by means of which readings to 3' of angle may be made. Provision must also be made for holding the bullet firmly in a horizontal position and for rotating it on its axis. The stage is provided with two movements, at right angles to each other, so the bullet can be centered in the field without difficulty. Microscope and stage are shown in Fig. 80.
The eyepiece is provided with a special „reticle" (or grid) which consists of a number of very fine lines which are exactly parallel to each other. The principle may best be described with the aid of a diagram. See Fig. 81. Actually the lines are much narrower than shown and they should be closer together. During the process of measurement the eyepiece containing the reticle remains in fixed position, unlike the Gaertner eyepiece already described.
The procedure is as follows. The bullet is placed in the holder on the mechanical stage and is rotated until a well-defined groove is located-one with sharp edges or with striations (bright lines) parallel to the driving edge on the groove. The bullet is then centered in the field by turning the knobs controlling the vertical and lateral movements of the object on the stage. The circular scale is now rotated until the bright line (or edge of the groove) is lined up exactly with one of the central parallel lines of the reticle and a reading is made, using the vernier to read the angle as accurately as possible. Since the mechanical stage does not (usually) have a micro-adjustment screw, as does the Gaertner protractor, precise settings are more difficult and, consequently, several should be made and averaged.
The microscope tube is now lowered until the edges of the bullet are brought into sharp focus. This does not change the angle, so no error will ensue. The bullet is now turned, by rotating the circular graduated scale, until the sides are parallel to the parallel lines of the reticle through which one sees the bullet. The suggestions made concerning proper illumination of the bullet (by throwing the edges into shadow against a white background) apply here as well as with the protractor eyepiece method. By moving the bullet laterally and at the same time adjusting the angle, it is possible to line up the bullet so that one of the parallel lines of the reticle will just touch the edge of the image of the bullet, thus insuring its being parallel to the same. A reading of angle is now made, and the operation is repeated several times and the results averaged. Now, the opposite edge of the bullet will be parallel with the lines of the reticle if the edges of the bullet are parallel. Unfortunately they often are not. To ascertain whether such is the case the bullet is moved laterally until this opposite edge of the bullet is precisely lined up with one of the parallel lines of the reticle. Several readings are taken as before and are averaged. If the average of the readings on the right hand edge of the bullet do not coincide with the average of readings taken on the left hand side, the results of the two series are averaged and this value is taken as the axis of the bullet. The difference in angle between the axis and the edge of the groove now having been determined, a calculation is made as to the length of barrel necessary to produce exactly one turn of the bullet. This calculation has already been explained. As stated previously, the principal difficulty in any method is that of determining the true axis of the fired bullet.
As an example of the application of this method a series of measurements were made on a bullet which had been fired from a .32 cal. Colt automatic pistol. The bullet was placed in a holder such as is used for the earlier forms of the comparison microscope and was centered so that when rotated it did so with a minimum amount of eccentricity. (The parallel lines of the eyepiece grid are helpful in this process of centering, as any eccentric motion is easily detected.) A groove (designated as No. 1) was then brought into proper position - i.e., in the center of the field, and the stage was rotated until one of the parallel lines of the grid coincided with the driving edge of the groove. Five such settings were made and the readings were averaged. Next the stage was rotated until the sides of the bullet were parallel with a pair of the lines of the grid. Five settings were made and the results averaged. This process was repeated for each of the six grooves-a total of 60 settings. The average of all of the angles was 3.36° or 3° 22'. The bore diameter of the gun being 0.305 inch, the length of barrel for one turn of the rifling in the gun would, therefore, be
A second and entirely separate series of measurements on the same bullet gave 16.4 inches for one turn of the rifling.
This illustration shows that when a near perfect bullet is at hand one can make measurements that have some meaning, but such bullets are too rarely encountered in actual cases.
To illustrate what may be done with this method on a fired lead bullet, under near perfect conditions, a .38 Spl. bullet was fired from a .38 cal. Colt Army Special revolver whose rifling was in excellent condition. The bullet was fired into cotton waste. Measurements made with the rifling meter had shown that the rifling in this gun had one turn in 16.0 inches.
Measurements of the angles made between the impressions produced by the driving edges of the lands and the nearly parallel sides of the bullet gave results from which a calculated value of 16.2 inches was obtained for one turn of the rifling. Such excellent values can be obtained under near perfect conditions-i.e., the bullet must be long enough to follow the rifling without slippage or eccentric motion, the impressions made by the driving edges of the lands must be long enough to permit accurate settings, the rifling must be in excellent condition so that clean-cut impressions will be made, and there must be no deformity of the bullet. In actual cases these conditions are rarely met, and the results will be only approximations-but, even so, may be quite useful.
Unless the true axis of the bullet can be determined, the results will be inaccurate. The measurement of angle on a single groove is meaningless. All must be measured and the average value used in the calculation. In the case of short bullets, particularly those that are conical in shape, measurements of rifling angle will have little meaning. The .380 ACP bullet affords a good example. The bullet is quite conical in form, it is relatively short, and the length of bearing surface on the rifling is very short. All attempts to determine the pitch of rifling on such bullets fired through barrels whose rifling pitch was known have been very unsatisfactory. The angles determined are too small and they differ from bullet to bullet fired from the same barrel. With lead bullets, having a larger area of contact with the rifling, much better results are obtained, as has been shown above.
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