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Ballistic Imaging (2008) / Chapter Skim
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2 Firearms and Ammunition: Physics, Manufacturing, and Sources of Variability
Pages 30-52

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From page 30...
... Modern firearms and ammunition manufacture relies heavily on the uniformity and interchangeability of component parts, yet each step in the production cycle presents an opportunity for microscopically fine differences from part to part. Likewise, the firing of a gun depends on the rapid and repeated performance of numerous mechanical steps that is designed to produce combustion, done in a controlled manner yet still not creating exactly identical conditions in repeated firings.
From page 31...
... Barrels Gun barrels are manufactured from solid pieces of steel whose composition is carefully selected for its chemical and metallurgical properties. A first step of the process, drilling, results in a comparatively rough hole of uniform diameter extending from one end of the barrel to the other.
From page 32...
... Other shooters believe a fast twist builds pressure and heat and they want a slow twist for minimum stability, and they have claims to back their theory." Some firearms differ from conventional rifling with square-edged grooves, using polygonal rifling instead. "Polygonal rifling has no sharp edges," and instead the raised lands in the barrel have a smooth, "rounded profile which can be difficult to discern when looking down the barrel.
From page 33...
... The shape of the firing pin hole for the Glock firearm indicates its characteristic rectangular firing pin. SOURCE: Excerpted from Tulleners (2001:Fig.
From page 34...
... ; they are semiautomatic in that they are self-loading but require separate, distinct trigger pulls to fire different rounds. "Automatic" is used to describe "a weapon in which the action will continue to operate until the force is removed from the trigger or the magazine is empty." Though a few fully automatic pistols have been marketed, they are rare "due to the near impossibility of controlling such a weapon [for accurate shots]
From page 35...
... Because these steps are performed very quickly, and with some speed and force, both the extractor and ejector mechanisms can leave marks on expended cartridge casings. 2–A.2  Ammunition Modern ammunition takes the form of integrated, self-contained cartridges, integrating three key elements in one unit: • a bullet, the actual projectile that is expelled from the firearm's barrel; • propellant, which generates the force and pressure needed to put the bullet in motion and into flight; and • a primer, which in modern usage is a volatile and pressure-sensitive chemical mixture that is responsible for igniting the propellant.
From page 36...
... Primer The use of a chemical primer to ignite the propellant dates back to the development of the percussion cap in the early 1800s, when it was discovered that striking a cap containing fulminate of mercury created a flame that could then move into the main charge of powder. Today, the exact chemical composition of primer mixtures can vary and remains proprietary.
From page 37...
... . Likewise, the material holding the primer -- either the cartridge brass of the rim in a rimfire cartridge or the cup in a centerfire primer -- must withstand the impact of the firing pin, the detonation of the primer, and the expansion of gas from the ignited propellant without rupturing.
From page 38...
... Within the same case, thickness must also vary in particular ways, tailored to suit various tasks: maximum hardness in the rim (of a centerfire cartridge) in which the primer cap is seated, medium hardness with good elasticity in the central walls of the case, and softest at the neck or mouth end where the bullet is seated.
From page 39...
... . In centerfire cartridges, where "grease grooves" are created in the case by knurling, the
From page 40...
... The pressure from the firing pin creates a dent in the primer surface of the cartridge; more significantly, it causes a small explosion, the heat from which passes through the hole in the primer cap and into the main body of the cartridge. There, the charge of powder burns rapidly in a confined space, converting from a solid to a gas and exerting great pressure against all surfaces.
From page 41...
... 2–C Basic Toolmarks on Ballistics Evidence 2–C.1  Cartridge Case Markings Breech Face Marks Gas pressure created during the firing process exerts pressure in all directions, including forcing the head of the cartridge against the breech face. Hence, the surface area of the cartridge head may pick up negative impressions of any linear striations or other features left on the breech face when it is filed and machined.
From page 42...
... documents the class of 9mm pistols for which the rotary cutting tool used in milling the breech face not only leaves distinctive arched markings that are impressed on the primer surface, but
From page 43...
... Lower pressure cartridges are not expected to consistently produce decent breech face impressions." He adds that cartridge chamber pressure, bullet weight, and primer hardness "can vary to such an extent that an examiner will not be able to identify test 1 to test 2 when different ammunition is used in the same gun;" hence, "one of the cardinal rules in firearm examination is to test fire the gun with similar ammunition as the evidence ammunition if at all possible" (Tulleners, 2001:3-3)
From page 44...
... Because drag marks may be repeated -- that is, they may be a function of the behavior of the firing pin in a particular gun -- they become important landmarks for traditional firearms identification and ballistic imaging alike, providing a benchmark to orient casings consistently. It is also important to note that the ­mechanics of firing is such that there is variability in the exact position where the firing pin impacts the cartridge across different firings; the pin may wobble slightly and strike at slightly different points and angles. In rimfire weapons, the firing pin strikes the brass of the outer rim of the cartridge head.
From page 45...
... In addition to the shape of the ejector mark and any individual scrapes or textures therein, ejector marks also serve the same important purpose as a firing pin drag mark: They provide a point of reference for proper orientation of cartridge cases relative to each other in comparison. Other Markings During the firing process, gas pressure works on all surfaces, forcing the material of the cartridge against the chamber of the weapon; particularly in semiautomatic weapons, other firearms parts are used to circulate ammunition through the weapon and eject spent casings.
From page 46...
... Heard (1997:131) concludes that "generally speaking it is possible, although extremely difficult, to match bullets from polygonally rifled barrels." 2–D  The Manufacturing of Firearms and Ammunition The underlying theory of firearms identification depends critically on manufacturing processes, positing that the tools used to form component parts wear with use so that each part may share the same gross features yet differ in microscopic (and, presumably, uniquely individual)
From page 47...
... That is, the process of mass-producing firearms is one that can be well partitioned: constituent parts of a new firearm can be drawn from large bins of fairly standardized parts and automatically fitted together with low yield loss, resulting in weapons of reasonably identical properties in terms of size, weight, and performance. Yet individual manufacturers differ on the exact steps used in machining and assembling firearms, and choices on the amount of filing or polishing to do on firing pins or whether to apply paint to the breech face can have an impact on the resulting toolmarks.
From page 48...
... The bullets exhibit slippage and/or extremely shallow land impressions that often make even shoulder location difficult to determine," and even "breech face marks are either non-existent or change from shot to shot." Collins' specific inquiry into the manufacturing pistol was based on attempting (unsuccessfully) to replicate crescent shaped marks observed in some firings, imprinted directly below the firing pin impression and believed to be caused by peening of the breech face surface under repeated firings.
From page 49...
... It is standard practice for manufacturers to apply a head stamp, engraved on the rim of the cartridge head, to identify the manufacturers and perhaps the specific make of the ammunition; they may also use colored paints or other indicia to differentiate between specific makes and calibers. Ammunition manufacturers also vary in some postprocessing steps, such as the application of a lacquer sealant to the primer surface.
From page 50...
... report finding distinct striated markings near the edge of the primer surface on a batch of 115 grain Remington 9mm Luger ammunition. The marks appeared to be due to manufacturing and not firing: when a pair of casings was rotated so that identifying marks in the firing pin impression were in the same orientation, the extractor marks on the cartridges also lined up but the newly found striated marks on the primer surface were 90 degrees out of alignment.
From page 51...
... Marks in this class include thick striation-like parallel marks across the primer sur face; these may obscure texture patterns in the breech face impression and may extend into the firing pin impression. Russian-made Wolf ammunition is well known for these marks, which have also been observed in other ammunition types.
From page 52...
... observed Russian-made ammunition with primers that, before firing, "had parallel marks like one might find as a result of breechface impressions." Finding that "the marks continue around the curve of the primer into the sides which were not visible," he concluded that "the only way that marks could have gotten there was by the rollers in the brass mill where the sheets of brass were made." The third class of marks, those that are problematic for comparison, include ammunition types with existing distinct parallel and cross marks on the primer surface, making it difficult to discern which textural features were created by firing. Murray (2004:314)


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