Bullet Comparisons
Many published studies have demonstrated that no two irearms produce the same unique marks on ired bullets and cartridge cases; this is even true with irearms of the same make and model. The machining of the manufacturing process combined with the use of the firearm leave surface marks on the metal parts of the firearm that are not reproducible in other irearms. These marks are transferred to the bullets and casings when discharged from the firearm. Because there is no practical method of comparing the striations on the inner surface of a rifled weapon with the striations on a fired bullet, reference bullets of the same make, style, and caliber must be created by iring them from the questioned firearm. Not only would cutting the barrel open be impractical, but the comparison would then be between positive (the barrel) and negative (the questioned bullet) impressions. The known ired bullets must be captured and preserved, however, so that they are as “pristine” as possible and not deformed or damaged. Firearms are typically discharged into a water tank where the water slows and eventually stops the bullet without altering its striations; other bullet recovery systems are used from the simple (a bucket filled with rubber shavings) to the high tech (sandwiched layers of specialized materials); Figure 1 shows some examples. The known bullet is then recovered, labeled, and used as a reference in the comparison; multiple known bullets may be created, if necessary. The questioned and known bullets are irst examined with the naked eye and slight magnification. The number of lands, grooves, their twist, and the bullets’ weights are recorded. Because these are higher-order class characteristics, any deviations from the known bullet indicate that the two bullets were fired from different barrels. If the lands, grooves, and direction of twist all concur, then the next step is microscopical comparison of the striations on the bullets. The comparison is performed using a comparison stereomicroscope with special stages that facilitate positioning the bullets in the focal plane and allow for rotation of the bullets on their long axis (see Figure 2). The bullets are positioned on the stages, one on each, both pointing in the same direction, and held in position with clay or putty; this allows for easy repositioning, and the soft material will not mark the bullets’ surfaces. The known bullet is then positioned to visualize a land or groove with distinctive striations. The questioned bullet is then rotated until a land or groove, respectively, comes into view with the same striation markings (see Figure 3). The lands and grooves of the two bullets must have the same widths. More importantly, the two bullets must not be merely similar, but they must have the same striation patterns with no significant differences. This last point is critical: Not only must forensic irearms scientists see the positive correlation between the significant information on the bullets’ surfaces, but they must also not see any unexplained differences. Each rile barrel is unique: No two of them will have identical striation patterns. This is true even for barrels that have been riled in succession, one after the other. It takes education, guidance, and mentoring to train a person’s eye and judgment on the subtleties of bullet striation patterns.
FIGURE 1 Test-firing bullets into a water tank preserves the striae on the bullets but also slows down the bullet so it can be safely discharged and retrieved. Water tanks can be difficult to maintain, however, and other methods have been devised. Some are simple, like a 5-gallon bucket filled with rubber shavings (a), and some are technologically complex, like the bullet recovery system made by Ballistics Research Inc., Rome, Georgia (b). This system uses two specialized types of material sandwiched in a series of alternating layers inside a caster-mounted metal box. Projectiles come to rest within the series of layers, where they are easily recovered by hand.
FIGURE 2 Much like the comparison microscope used for hairs and fibers work, the firearms comparison microscope optically joins two stereomicroscopes. This allows forensic scientists to view two objects simultaneously side by side.
FIGURE 3 A positive association between bullets must have the same land and groove patterns, widths, and striation patterning with no significant negative correlations. A negative association between bullets would demonstrate numerous significant misalignments of striations with no significant correlation of striae.
Recent research has indicated that bullet and cartridge case comparisons are accurate and very reliable. Studies show that the overall error rate for cartridge case comparisons is on the order of less than 0.1%; false positive rates are about 1.0% and false negative rates are around 0.36%. For bullets ired from consecutively manufactured barrels, the error rate is less than 0.1%. By comparison, a home pregnancy test conducted by a medical professional has an overall error rate of 2.6%. Bullet striation comparisons are difficult enough with intact, clean bullets; in reality, most bullets recovered from crime scenes are mangled, deformed, and dirty. Often only a small portion of a bullet may have useful striations for comparison (see Figure 4). The barrel may not have been cleaned recently, and rust, grit, and built-up residues may have been transferred to the bullet when it was fired.
FIGURE 4 This lead bullet has deformed and separated from the copper jacket. It would be difficult, if not impossible, to find useful, suitable striations on a corresponding known bullet.
