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Criminalistics Laboratory Methods

Surgical pathology description of bullets

Each bullet keeps a diary in its own way of where it has been and what it has done. Now that you understand the function of a bullet, many of these changes become easy to interpret. The bullet base will contain irregular dimples marking the pressure delivered there in its acceleration. the bullet sides will bear the markings of the barrel interior rifling. These spiral lines, or striae, contain the micrscopic imperfections of the gun from which it was fired and can be as specific as a fingerprint. The bullet nose carries information about the target, and recognizing these may give a clue to the injury rendered.

Remember in measuring bullets to determine the type of cartridge used that the actual bullet diameter, even of non-deformed bullets, is not the same as the name of the cartridge. Most names have a historic basis and have little to do with any real physical measurements: a .30-06 was named for a .30 caliber cartridge developed in 1906; the handgun cartridges called .357 magnum, .38 special, and 9 mm parabellum have essentially the same .357 inch actual diameter. Therefore, use caution in opinions regarding the type of weapon or cartridge used based upon examination of bullets.

The best surgical pathology description would give dimensions as measured (use vernier calipers for best results), shape, and appearance of surface. Photography will be valuable.

Expansion of a semi-wadcutter hollowpoint bullet increases the frontal area and blunts the shape. The degree to which this happens depends upon the texture of the tissue impacted, the velocity at impact, and the softness of the bullet (usually quite constant). With the exceptions of lung and bone, tissue densities are relatively constant. Velocity is the most important factor.

No change in shape occurs until impact velocity achieves about 800 fps. Between 800 and 1000 fps a slight flattening of the bullet nose can be expected. Over 1000 fps real expansion starts to occur and by 1200 fps the nose is turned over to form a mushroom shape. An interesting artefact of impacts around 1000 fps is the tendency of the copper jacket to be shed from the lead. The jacket stops in the subcutaneous tissue and the bullet will continue to penetrate. This accounts for fragments of copper (with rifling marks) commonly seen as surgical specimens. At velocities approaching 1500 fps the bullet is transformed into a rounded ball of lead and copper. The above results are uniformly valid only in artificial media (such as ordnance gelatin) but correlate with human tissue. Examples follow on the next page:

The soft exposed lead nose on non-full metal jacketed bullets can be imprinted with anything that is penetrated by the bullet. Wood, glass, fabric, plastic, or tissue may leave marks as well as fragments on the bullet tip. Bone struck by bullets may not only fragment the bone, but also split the bullet. Lead round nose bullets can penetrate deeply and strike bone at relatively high velocity and can be cleanly cut in half or shaved vertically. Full metal jacketed round nose bullets are less affected, but are often irregularly flattened upon striking bone. Bullets that come to rest in soft tissue without striking bone are often intact.

Intermediate targets, such as glass, wood, clothing, or even paper, may influence the path, shape, and fragmentation of projectiles. Such factors must be taken into account in the recovery of evidence. (Stahl et al, 1979) Even tempered glass, which shatters and fragments easily on impact, may deflect handgun bullets (low velocity) significantly. High velocity, jacketed bullets will be deflected much less. (Thornton, 1986)

Flattening of shotgun pellets may not necessarily indicate a close range contact with a target, as the pellets may be deformed on firing. Recently developed shells use plastic packing materials and plastic capping to diminish deformation. (DeMuth et al, 1978)

Even pellets of air guns may show characteristic striae (Cohle et al, 1987). Silencers used over the muzzle of a gun are often misaligned and can produce characteristic striae. (Menzies et al, 1981)

Examination of whole bullets and cartridge cases

If a bullet is recovered from the scene or from the body, it may be compared to bullets obtained by test-firing the suspected weapon. Test firing is done using similar ammunition. Bullets are marked on the nose at the 12 o'clock barrel position (called "index", "witness", or "reference" marks). Consecutive test bullets are then fired into a water tank, recovered, and juxtaposed with a comparison microscope to compare test bullets with the recovered evidence. Index marks help to align test bullets to determine reproducibility of markings. Photographs should be taken (a ruler or coin can be used to give scale and alignment).

Comparison of bullets involves "class" and "individual" characteristics. These characteristics are based upon "striae" left on the bullet as it passes through the barrel.

Class refers to the type of caliber and rifling. Rifling pattern may turn to the right or left, with a given rate of twist. The number and depth of grooves can vary also. Some newer guns use "polygonal" rifling resembling the reversed image of a twisted square rod. A particular type of gun (.38 Smith and Wesson, or 9 mm Glock) will impart these class characteristics.

Individual characteristics are used to try and determine if a specific gun (say one of many 9 mm Glocks) was used. These individual characteristics are based upon burrs or imperfections in the barrel, particularly the muzzle, that impart specific markings, or striae, to fired bullets. If such markings are present, they may lead to a "determinative" identification. In general, smaller caliber weapons (.22) yield fewer reproducible characteristics in fired bullets than weapons of larger caliber (.45).

In the image below, two sets of bullets of the same class are roughly compared to indicate how difficult this can be when bullet deformation is present.

Patterns of Striae on Bullets

A system has also been described for identification of jacketed sporting rifle bullets using twelve parameters:

  1. Identification number
  2. Manufacturer
  3. Weight
  4. Diameter
  5. Cartridge
  6. Base design
  7. Length of bearing surface
  8. Color
  9. Shape
  10. Location and description of crimping cannelure
  11. Location and description of other cannelures
  12. Miscellaneous notes.

Such parameters may aid in narrowing the search for suspected weapons or ammunition. (Booker, 1980)

Optical devices for identification of bullets and tool marks include microscopes with cameras. Standard light microscopy has limits of resolution defined by magnification and illumination. Digital cameras are limited by number, color, and density of pixels detected. Confocal microscopy provides a means for obtaining information regarding depth in an image. (Banno, Masuda, Ikeuchi, 2004)

There are three results of comparison identification. Test fired and recovered bullets can: (1) be related to the same weapon; (2) be unrelated to the same weapon; (3) not be compared with this type of examination. Conclusions should not be based upon probabilities in test firing. Image analysis can be employed to assist the process of bullet comparison and identification of the weapon used to fire the bullet. (Brinck, 2008)

Criteria for consecutively matching striae (CMS) have been established. Bullet striations are typically three dimensional because there is depth and contour imparted in a deformable metal such as lead. Matching of these three dimensional toolmarks is based upon the presence of at least two different groups of at least three consecutive matching striae that appear in the same relative position, or one group of six consecutive matching striae, oompared to a test toolmark. (Chu et al, 2012)

In many situations, however, the hospital pathologist as medical examiner will not be involved with test firing. The hospital pathology department may receive bullets or bullet fragments from patients. Such evidence should be clearly identified, with a "chain of custody" followed. The pathologist will dictate a report and release the evidence back to the authorities.

Every firearm that is fired imparts a set of physical markings to the fired bullet and cartridge case. Components of the firearm that produce these unique characteristics are: firing chamber, breech face, firing pin, ejector, extractor and the rifling of the barrel. These unique characteristics assist forensic scientists in determining what firearm was used to fire the bullet. Characteristics transferred to the cartridge case include: firing pin impression, center of firing pin impression, and ring of firing pin impression. In one study these features could correlate the cartridge case to the firearm 96.7% of the time. (Md Ghani et al, 2010)

Examination of bullet fragments or bullet composition

In many cases, recovered bullets will be too deformed for comparison studies. The "lead" of bullets actually may contain up to 26 common elements, of which up to 12 can be used for differentiation. One of the most common of these is antimony (1 to 2%) Unfortunately, bullets within a box or lot do not have uniform composition, but there may be distinct groups of bullets within a box. (Haney and Gallagher, 1975) (Koons and Grant, 2002)

When analysis of the bullet lead is necessary, but a copper jacket is present, the copper may be most efficiently removed, without contamination of the lead, by use of concentrated nitric acid. (Izak-Biran et al, 1980)

Detection of the type of bullet (jacketed or not) may be done by a dithiooxamide (rubeanic acid) test. This test detects copper and nickel, which may be components of jacketed ammunition, on the target. The rubeanic acid forms a dark green precipitate in the presence of copper, pink or blue with nickel, and brown with cobalt. Blood and other materials on the target produced false negatives. (Lekstrom and Koons, 1986)

Bullet particles may also be detected in bone fragments from skeletal remains when no soft tissues remain. After determining that radiopaque particles are present, surfaces of the bone fragments containing the particles can be exposed by cutting. The surfaces can then be analyzed by SEM-EDA and by electron probe microanalysis to identify lead (Pb) and antimony (Sb). The electron probe technique aids in differentiating antimony from abundant calcium of bone. (Simmelink et al, 1981) Detection of bullet lead has also been carried out with proton-induced X-ray emission (PIXE) analysis, even in a victim buried for several years (Fischbeck et al, 1986).

Even if an exit wound is present, a search for bullet fragments or jacket material should be done, with radiographs if necessary. A new type of ammunition, Winchester Western Silvertip, may pose a problem, as its aluminum jacket is only faintly radiopaque (Conradi, 1982).


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