The same two Bullets shown in Plate XXX, but as viewed with the
A portion of the right-hand bullet appears in the right-hand half of the field, and a portion of the left-hand bullet in tbe left half. The striations on the two bullets match exactly nickel-jacketed bullets in Plate XXVI. In both cases the engraving consists of deep major furrows with fine striations in between these major furrows.
On each fired bullet the deep furrows have been engraved by the lands, and the striations in between these major deep furrows by the tool-marks in the bottoms of the grooves.
It will be noted that only the lower portion of each fired bullet is engraved. This is only to be expected, as the parallel part of the bullet is the only part which takes the rifling, the tapered nose being of too small a diameter. But it will be seen that upper portion of the land furrow in the lead bullet has the appearance of being "doubled," this "doubling" decreasing as it approaches the base.
This is a very common feature in fired bullets and is caused by the bullet failing to enter the bore absolutely nose on. The result is that the forward part of the parallel portion of the bullet takes the rifling, and is engraved accordingly. But as the bullet travels farther into the bore, and the whole of the parallel portion enters the bore, any tendency to oblique movement is checked, and the bullet is forced into the rifling with its longitudinal axis coincident with the axis of the bore. When the bullet is so forced to assume true nose on movement it skids from its initial slanting direction and takes the rifling correctly in a manner similar to that in which the wheel of a car which is travelling just off the direction of a tram-line will skid into that tram-line and travel along it. But the mark of the first impact with the rifling remains, and constitutes what can best be described as a "Skid Mark" at the front end of the land furrow.
If the chamber is very closely bored, and the leed, or chamber cone connecting the chamber with the rifling, is short, and the bullet is a tight fit, the bullet will enter the rifling absolutely nose on. In this case there will be no skid-mark when the bullet is recovered after firing. But in revolvers in which the cylinder may not revolve so as to bring the chamber in absolute collimation with the bore; and in cheap self-loading pistols in which the chamber may be on the loose side and the leed rather long, skid-marks are far more frequently present than absent. They mean nothing and are of no utility whatever in helping to identify an individual weapon, as they are not by any means always present on bullets fired from the same weapon. For bullets vary slightly in diameter, and a large bullet may easily escape any sort of initial wobble and enter the rifling nose on, while a small bullet may enter the rifling at a slight angle. In such circumstances the large bullet will have no skid-mark and the small bullet will have such a mark, in spite of the fact that both were fired from the same weapon.
There are various types of rifling, and these types differ widely. For example they vary in the number of grooves; in the relative width of lands and grooves; and in the direction of the twist, that is the twist may be right handed or left handed.
The rifling used in revolvers and self-loading pistols may be divided conveniently into the following five types—
SteyrType.—Four grooves; right-hand twist; grooves and lands of equal width. Used in all earlier self-loading pistols, such as the Borchardt.
Browning Type.—Six grooves; right-hand twist; narrow lands and broad grooves. Much the most common.
Colt Type.—Six grooves; left-hand twist; narrow lands and broad grooves. Used in all Colt revolvers and self-loading pistols, in Bayard pistols and Spanish copies of Colt pistols.
two different '32 lead revolver bullets both fired by the same revolver
These bullets are the same as those which were shown in Plates XXX and XXXI, and are reproduced again in order to show how photo-micrographs should be prepared as evidence. This Plate should be studied in conjunction with Plate XXXIII
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