As has already been pointed out, workers in the field of firearms identification often have need of rifling data not presently available to them, either in textbooks, reference books, or from firearms manufacturers. Not infrequently a bullet is brought to the laboratory and the examiner is asked what kind of a gun should be looked for, there being no gun or suspect and perhaps even no known motive for the crime that has been committed. In such a situation it is obvious that the investigation of the case might be much simplified if the authorities could be given some information concerning the type and the probable (or even possible) make of the gun from which the evidence bullet had been fired. Any narrowing down of the search for the gun is clearly of assistance. If it could be said positively that the bullet had been fired from a .32 caliber automatic of a particular make, the investigators could concentrate their efforts and avoid much loss of time and effort. Unfortunately the usual actual case is not quite so simple, because different manufacturers often have specifications (or at least procedures) which result in „class markings" that are much alike. But, at least, the search can be narrowed down considerably if proper information is available to the expert. In the author's experience such information has been very useful to the investigating officers on several occasions, and in one instance it lead directly to the solution of a murder case in which there was no suspect or known motive.
It might appear that the rifling specifications information might be obtained from the manufacturers, and as a matter of fact many manufacturers do have information not only as to present but also past rifling specifications and most of the larger ones will furnish the expert with such data. Also it may be said that these larger, and usually (though not necessarily) responsible, manufacturers do make a sincere effort to follow their own specifications. If each and every manufacturer (past and present) had different specifications and if these specifications were followed at all times, then by making a few observations and measurements on a fired bullet one would be able to say with a considerable degree of certainty what make of gun had been used.
But the case is not so simple. In the first place many of the cheaper foreign guns, of which there are probably hundreds of thousands in this country, are „assembled" guns, i.e., the various parts, which may have been made at different shops and under no rigid system of control, were bought up and assembled by a „manufacturer" or dealer who put his own name on the assembled job. In Spain there were a number of shops that devoted themselves exclusively to the rifling of barrels. Some of them had specifications which were followed, more or less, while others seem not to have had any specifications. Another difficulty is that even the reputable manufacturers do make changes in their specifications from time to time, just as typewriter manufacturers change their „fonts," i.e., designs of letters, from time to time. This, however, would be a minor difficulty if the expert had the necessary information as to these changes and when they were made. Actually under certain circumstances these changes may be very useful as they may help to „date" a gun, but this is only so in case the expert does have the information. Often the manufacturers themselves have no record as to when a change in specifications was made or when it went into effect in their shop practice. One of the world's largest manufacturers has on three occasions had all of its records destroyed, twice by fire and once by flood. In Europe, because of wars and revolutions, valuable records have been destroyed, intentionally in many cases.
In an article published a number of years ago (2) the author stated his intention of making a series of measurements of rifling characteristics of all of the firearms that became available and described in that article the instrument that had been designed and built for the purpose of making accurate measurements of the angle of twist of the rifling in firearms. Because of the enormous number of rifled firearms it has been necessary, as already mentioned, to limit the measurements to hand guns, though it would be of value to have a similar study made on rifles.
The „rifling meter," as we have chosen to call the instrument we have developed for measuring the degree of twist of rifling in a barrel, is shown in Fig. 75. A lead disk, of diameter slightly greater than the bore of the gun, is pushed through the barrel in such manner that it will follow the rifling, and the amount of rotation of the disk is measured at any desired interval by reading the angle on the graduated circle at the interval selected. The lead disk is mounted on the end of a „push rod," the other end of which is inserted into the „hub" carrying the graduated circle and is locked in place. The circle itself is mounted on sturdy ball bearings so that it can rotate freely. After the disk is inserted into the end of the barrel, by turning the hand wheel at the extreme right, a „zero" reading is taken. Then the disk is pushed forward an exact, predetermined amount and the rotation of the disk is determined by taking a reading of angle on the graduated circle and subtracting from this value the original „zero" reading. For example, assume that the „zero" reading (usually made about 1/4 to 1/2 inch from the end of the barrel) was 8.75° and that the reading after the disk had been pushed forward exactly 1/a inch was 20.00°, the difference of 11.25° represents the extent of rotation of the rifling in 1/2 inch of barrel length. Then, for 1 inch it would be 22.50°; and 360/22.50 =16.00 inches, the number of inches required for one complete turn of the rifling. The graduated circle has a vernier which enables one to read to 0.05° (or 3' of angle). The graduated drum, just to the left of the hand wheel, is graduated into 100 equal divisions, each representing 0.001 inch. So, one rotation of the wheel causes the disk to move forward 0.1 inch. The accuracy of the actuating screw is such that in 10 inches of thrust the error is less than 0.001 inch.
In measuring the rifling of a barrel a series of readings at definite intervals is made in order to see whether the twist is uniform throughout. For a very short barrel the interval chosen would be 0.1 inch, for those 3 to 4 inches in length, 0.2 inch, and for a 6- or 7-inch barrel the interval might well be 0.5 inch. The reproducibility of measurements on barrels that are well rifled and in good condition is very satisfactory. Examples of reproducibility will be given later.
The instrument will now be described in somewhat greater detail especially for the benefit of anyone who cares to have one made. It consists of three essential parts, and for clarity each will be described separately.
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