While there are many questions that come up in firearms investigations, the two that come up most frequently are: (1) What kind of a gun was used, and (2) was this particular gun used? Both of these questions involve a study of the markings which are left on the fired bullet or cartridge, or both. On each there will be two types of markings: repetitive and accidental. The accidental markings may have some relation to the investigation (instances of which will be mentioned later) but are of no value in identifying a particular weapon or make and model, since they are not formed regularly in the operation of the gun. Repetitive marks, on the other hand, are very useful because they show identity of performance. Experience has shown that no two firearms, even those of the same make and model and made consecutively by the same tools, will produce the same markings on a bullet or a cartridge. There will, of course, be a „family resemblance"-e.g., the bullets will have (approximately) the same diameter, same number and widths of grooves, same pitch and direction of slant of rifling marks. Technically expressed, the guns have the same „rifling characteristics," but, while the markings may be sufficiently alike to characterize the make (and even model) of the gun, they are not sufficiently alike as to be considered „identical" and are not likely to confuse an expert. These „Class Characteristics* have now been measured for a very large number of guns and the results are set forth later.
*The words identity and identical as used in firearms investigations do not mean that the markings on two bullets two objects are ever identical in the absolute sense. just as no or on two shells are absolutely alike in every particular. No two persons are alike, no two objects made by man or by nature are absolutely alike. So the term „identity" is a relative one.
On the other hand, bullets fired through the same rifled barrel and cartridge cases (usually called shells) fired in the same gun may be expected to show an „identity" of markings which is peculiar to this particular firearm and to no other.
These markings serve then to identify a particular rifled barrel because that barrel has an individuality possessed by no other barrel. In 1926 at Springfield Arsenal a very interesting and conclusive experiment was made. Four barrels were rifled one after the other with the same rifling tools in an attempt to produce barrels as alike as possible. Bullets were fired through each barrel and compared. It was found that no two barrels matched completely; each had a distinct and separate individuality. Some time later Goddard fired 500 rounds through a machine gun and found that even bullet No. 500 could be matched with bullet No. 1, indicating that the individuality of a barrel persists. The results of these two early experiments have been confirmed over and over again in identification practice and are now generally accepted. Similarly, the markings produced on the head of a fired cartridge (shell) often can give valuable information as to the type and make of gun used and often can identify the particular gun when located.
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