The depth of the grooves on a fired bullet has little application to the problem of identification of a firearm. While it is fairly easy to determine by measurement the depth of the grooves in a rifled barrel it is very difficult to measure the depth of a groove on a fired bullet, and even where measurements are possible they ordinarily have little meaning. If all fired bullets were undamaged and perfectly symmetrical and if each represented precisely the cross section of the barrel, measurements could be made and they would have some significance. But these bullets do not exist. No two bullets, even when fired successively, will be the same in every particular.
Very seldom does a bullet fill completely the cross section of the barrel through which it is fired and unless the metal of the bullet is pressed to the very bottom of the groove even an accurate measurement on the bullet would have no significance. In practice it is found that a series of measurements of groove depths made on different grooves of the same fired bullet will show differences and since the characteristic which is being measured is a matter of only a few thousandths of an inch the variations are proportionately large. The majority of bullets brought to the laboratory show some degree of mutilation and many of them show extreme mutilation.
Different makes of guns do have a considerable variation in groove depths despite statements and inferences to the contrary in some textbooks, and these differences have some qualitative significance. Groove depths may vary from 0.002 to 0.010 inch, and even more in some of the older guns. Here again manufacturers lay down a specification as to the depth of grooves. One manufacturer specifies the same groove depth for over 30 models of all calibers made from .22 to .45. Actually these specifications are not strictly followed, and probably there is no reason why they should be.
The statements made above may seem to present a very discouraging picture as to the value of class characteristics, though such is not intended. As a matter of fact; class characteristics are often very useful indeed, but all are by no means of equal value. Caliber (bore diameter), number of lands and grooves, and the direction of the twist are the ones most often used. Widths of grooves on the bullet (indicating the widths of lands in the gun) could probably be used more than they are. If one has no equipment for making precise measurements he can at least use the alternate method, described later, which requires only a stage micrometer slide and the comparison microscope. In the use of class characteristics to locate a gun one must be careful not to go too far in his predictions as to the make and model of gun that must be found. The fact is, as will be seen by examining the data presented in a later section, there may be several makes and models of firearms that have rifling characteristics that are nearly alike. If, for example, one has a bullet with characteristics indicating that it was (or could have been) fired from a Smith and Wesson, he will be wise in saying that the bullet was fired from an S&W or from some other gun having similar rifling characteristics. In firearms investigations one should be very careful not to make positive statements that may backfire.
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