Still another method was studied and the results were quite successful. Instead of making settings on the sides of the bullet, in the manner already described, to determine the axis of the bullet, a procedure of determining the „axis of rotation" was devised and the angles that the driving edge of the land impressions made with this „axis of rotation" were measured and averaged.
The bullet is mounted as before and is centered in the same way. Then a fine straight quartz fiber (ca. 0.005 inch in diameter) is fixed in position above the bullet so that it is exactly in line with the axis of the screws which hold the bullet in position. This is used as the „reference axis" and, of course, remains in fixed position. The angles made by the driving edges of the lands with reference to this „axis of rotation" are now measured and averaged on the theory that i f the „axis of rotation" (i.e., the reference axis) and the true axis of the bullet do not exactly coincide the errors will cancel out i f measurements are made on all of the grooves on the bullet. Fig. 82 shows the quartz reference fiber mounted in the axis of the bullet holder.
After centering the bullet in the optical field and bringing the quartz fiber into sharp focus, the mechanical stage, upon which the bullet and holder are mounted, is rotated until the quartz fiber is lined up so that it coincides exactly with one of the parallel lines in the grid in the eyepiece, and a reading is made on the graduated circle of the stage. Several settings (usually five) are made and the average reading is recorded as the „axis of rotation" or reference axis.
The land impression is now brought into sharp focus and the driving edge of the land impression is brought into exact coincidence with one of the parallel lines in the grid in the eyepiece, and a reading is made on the graduated circle of the stage. Several settings (usually five) are made and the readings averaged. The difference between the averaged values of the two sets of readings indicates the angle or slope of that particular land impression with the „axis of rotation." This process is repeated for the driving edge of each land impression on the bullet and the average of all angles is used in the computation of the length of barrel required for one complete turn of the rifling. Readings on less than all of the land impressions present on the bullet will not suffice. A single reading of rifling angle has little if any value and may be very misleading.
Instead of using fired bullets to test this method it was tried out under more nearly ideal conditions. Three bullets (one Lubaloy coated, the other two of plain lead) were pushed through the barrel of a .38 cal. Colt revolver, using the rifling meter which has already been described. The rifling in this gun was in excellent condition and repeated measurements of the pitch of rifling gave 16.0 inches as the length of barrel required for one turn.
The measurements of pitch of the rifling on the three bullets used in the test gave 15.9, 15.9, and 16.0 inches, thus supporting the theory and showing what can be done with „perfect" bullets.
The accurate measurement of rifling angles on bullets requires not only near perfect conditions, as already indicated, but it also requires equipment not found in the average police laboratory. Furthermore, it requires familiarity with these special instruments and a high degree of skill in their use. Finally it is a time-consuming and tedious procedure. For these various reasons it is not likely that measurements of rifling pitch will ever come into common use in firearms identification laboratories.
Fig. 83 shows an instrument designed and marketed by Leitz for the measurement of rifling angle on fired bullets. The author has had no experience with it.
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