2
Firearms and Ammunition: Physics, Manufacturing, and Sources of Variability

A firearm is a dynamic system for delivering maximum destructive energy to a target, in the form of a high-velocity bullet, with minimum delivery of energy to the shooter. To that end, the firing of a firearm and the subsequent generation of ballistic toolmarks are the end results of processes that are simultaneously characterized by high uniformity and great variability. 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.

In this chapter, we summarize the basic parts of firearms and ammunition (Section 2–A) and describe the physical processes that take place when a trigger is pulled and a gun is fired (2–B). These sections are not intended to be comprehensive examinations of the history and features of firearms and ammunition nor a complete catalogue of firearms products in current use. Rather, they provide context for the principal focus of this chapter: describing the types of toolmarks left on ballistics evidence by firing (2–C), particularly those that are typically imaged and input into ballistic image databases.1 We close in Section 2–D with brief descriptions of concepts in the manufacture of both firearms and ammunition. A general understanding of manufacturing is essential not only for an appreciation

1

More detailed information and images are available at http://www.firearmsid.com.



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2 Firearms and Ammunition:   Physics, Manufacturing, and   Sources of Variability A  firearm  is  a  dynamic  system  for  delivering  maximum  destructive  energy  to  a  target,  in  the  form  of  a  high-velocity  bullet,  with  minimum  delivery of energy to the shooter. To that end, the firing of a firearm and  the subsequent generation of ballistic toolmarks are the end results of pro- cesses that are simultaneously characterized by high uniformity and great  variability. 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 differ- ences 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. In this chapter, we summarize the basic parts of firearms and ammu- nition  (Section  2–A)  and  describe  the  physical  processes  that  take  place  when  a  trigger  is  pulled  and  a  gun  is  fired  (2–B).  These  sections  are  not  intended to be comprehensive examinations of the history and features of  firearms  and  ammunition  nor  a  complete  catalogue  of  firearms  products  in current use. Rather, they provide context for the principal focus of this  chapter:  describing  the  types  of  toolmarks  left  on  ballistics  evidence  by  fi   ring (2–C), particularly those that are typically imaged and input into bal- listic image databases.1 We close in Section 2–D with brief descriptions of  concepts in the manufacture of both firearms and ammunition. A general  understanding  of  manufacturing  is  essential  not  only  for  an  appreciation  1  ore detailed information and images are available at http://www.firearmsid.com. M 0

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 FIREARMS AND AMMUNITION of the sources of variability in ballistic toolmarks, but also in assessing the  feasibility of implementing technologies like wide-scale ballistic imaging or  microstamping. 2–A ANATOMy OF FIREARMS AND AMMuNITION 2–A.1 Firearms Firearms come in a wide array of designs and specific makes, and each  represents a complex assemblage of numerous constituent parts. In this sec- tion we focus on the parts most central to the basic firing assembly since  the interest is in toolmark creation. Due to their widespread use in crime,  we also discuss some terminology in the specific context of handguns, as in  differentiating between revolvers and pistols. barrels Gun barrels are manufactured from solid pieces of steel whose compo- sition 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. Next  the barrel is bored with a reamer, designed to produce as smooth a surface  as possible on the inside of the barrel. The interior surface or bore bears  numerous scars and scratches from this drilling process; it is these random  imperfections—more so than subsequent steps—that are said to account for  individual characteristics on fired bullets (Heard, 1997:124–125). Barrels are further subjected to a rifling process, creating a pattern of  grooves on the inside the barrel. This rifling is essential to the firing accu- racy  of  the  weapon;  as  it  is  forced  out  of  the  barrel  by  gas  pressure,  the  bullet  impacts  with  the  barrel  rifling  and  is  given  a  rotation—somewhat  akin to the spin on a thrown football—that gives the bullet a more direct  flight. Some weapons, typically shotguns, have no rifling (“smoothbore”).  Most handguns and rifles have a spiral pattern of rifling to improve their  accuracy. The rifling may be created by forcing a carbide button through  the  reamed  barrel;  it  is  the  normal  wear  on  this  button,  as  many  riflings  are performed, that is said to impart individual microscopic variability in  markings in the barrel (along with residual scars or imperfections from the  original drilling). Additional steps in the process to finish a barrel include  heat treating (to impart hardness) and cleaning. Across  manufacturers,  barrels  can  vary  in  two  fundamental  features,  each of which are basic class characteristics (see Section 3–B.1). The first  is  the  direction  in  which  the  grooves  in  the  barrel  twist,  whether  left-  or  right-handed. Most U.S. makers use a right twist, although Colt revolvers 

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 BALLISTIC IMAGING are known for their left twist (Rinker, 2004:128). The second is the number  of grooes that are cut into the barrel—normally at a depth of 0.004–0.006  inch—to create the rifling, and, correspondingly, the number of raised lands  between  those  grooves.  Historically,  “no  standard  was  established  and  makers used, normally, six, seven, or eight grooves”; this remains the usual  range, although firearms have been fielded with as few as 2 and as many as  24 grooves (Rinker, 2004:130, 131). Barrels also vary in the degree of twist in the rifling, which affects how  much rotation is put on bullets as they pass through the barrel and exit.  Rinker (2004:127) observes that “few people agree on what is the proper  twist. Some people want an over stabilized bullet from a fast twist. They  claim best accuracy at all ranges. 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.  This type of rifling is almost exclusively manufactured using the hammer  or swage process” (Heard, 1997:123).  Chamber, breech Face, and Firing Pin The rear section (away from the muzzle) of the barrel bore is known  as the chamber; it is designed and sized to fit a specific caliber of cartridge  (see Section 2–A.2). The part of the firearm against which a cartridge sits  when it is placed in the chamber is the breech, and the whole assembly may  be referred to as the breechblock or breech bolt.  The  specific  surface  of  the  breech  that  makes  contact  with  the  base  of  the  cartridge  is  the  breech  face;  Figure  2-1  depicts  the  breech  faces  of  two firearms. The exact steps used to form the breech assembly can vary  by manufacturer, and the breech face may vary in terms of the amount of  filing or polishing done on it and whether any paint or other materials is  applied to it. Basic filing can create gross striation marks in linear arrange- ments; in others, a rotary milling operation may be applied to the breech  face surface, creating a pattern of concentric circles (American Institute of  Applied Science, 1982:77). These steps are crucial to the theory of firearms  identification as it is random imperfections created in these machining and  filing processes that is said to make the surface (and the negative impres- sions of said surface, left on fired cartridge casings) unique. A hole drilled through the breech assembly holds the firing pin, a very  hard steel rod that can be forced to protrude from the breech to strike the  primer of a cartridge seated in the chamber. While most firing pins have a 

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 FIREARMS AND AMMUNITION FIguRE 2-1  Breech faces with firing pin holes: Two firearms. NOTES: The top image is the breech face of a Smith & Wesson firearm; the bottom  image is the breech face of a Glock firearm. The shape of the firing pin hole for the  Glock firearm indicates its characteristic rectangular firing pin. SOURCE: Excerpted from Tulleners (2001:Fig. 3-3).

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 BALLISTIC IMAGING small rounded end or nose, some have more distinctive shapes; in particular,  Glock firearms are known for a rectangular firing pin. Firing pins are gener- ally made on a standard screw machine. Like the breech face, the tip of the  firing pin is subject to machining and filing steps that impart microscopic  imperfections. Revolvers and Pistols Handguns may be divided into two basic types—revolvers and pistols— by  the  manner  in  which  ammunition  is  loaded  and  cycled  through  the  firearm.  In  a  revolver,  “the  supply  of  ammunition  is  held  in  a  cylinder  at  the  rear of the barrel with each round having its own chamber,” and a ratchet  mechanism is then used to cycle the cylinder to the next position (Heard,  1997:18).  Revolvers  may  be  further  subdivided  by  the  manner  in  which  this cycling is performed. In single-action revolvers, the shooter manually  cocks the hammer, pulling it back and setting the ratchet action in motion.  A  trigger  pull  then  causes  the  hammer  to  drop  and  commence  the  firing  process. More complex—and more common—double-action revolvers save  a step: “A long continuous pull on the trigger cocks the hammer, rotates the  cylinder, then drops the hammer all in one operation” (Heard, 1997:18).  By  comparison,  pistols  are  self-loading,  making  use  of  ammunition  “contained in a removable spring-loaded magazine housed within the grip  frame.” Pistols have a single chamber, and individual rounds of ammunition  are  cycled  into  the  chamber  by  mechanical  means;  pulling  back  the  slide  rearward until the breech face is behind the top round in the magazine, and  then releasing it, forces the round forward and into the chamber for firing.  After firing, the spent cartridge case is ejected “through a port in the side,  or  occasionally  top,  of  the  slide.  At  the  end  of  its  rearward  motion,  the  spring-loaded slide moves forward[,] stripping a fresh round off the top of  the magazine and feeding it into the rear of the barrel” (Heard, 1997:19). Pistols  are  often  referred  to  as  semiautomatic  pistols  (or  semi- automatics);  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 oper- ate 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]. . . . Each shot causes the barrel to rise during recoil and before the  firer has time to reacquire the target within the sights, the next round has  fired”; consequently, “even at close range it is unusual for more than two  shots to hit a man-sized target” (Heard, 1997:17, 18). For  the  objective  of  the  recovery  of  ballistics  evidence  and  imaging 

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 FIREARMS AND AMMUNITION thereof, the distinction between revolvers and pistols is vital: while pistols  forcibly  eject  spent  rounds,  revolvers  do  not.  Hence,  casings  may  only  be  recovered  at  a  crime  scene  involving  a  revolver  if  they  are  specifically  e   mptied by a shooter (e.g., for reloading). Extractor and Ejector Both revolvers and pistols make use of an extractor, typically a small  arm that fits over the rim of the cartridge. As the name implies, the extrac- tor serves to pull a spent cartridge from the chamber so that a new cartridge  can take its place. In a revolver, the extractor—which can remove all car- tridges simultaneously by depressing the ejection rod (or extractor rod)— also has ratchet notches that advance the cylinder to the next chamber. In a  semiautomatic pistol, however, the extractor removes the cartridge so that  it makes contact with the ejector, typically a fixed protuberance that strikes  the rim of the cartridge. 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 car- tridges, 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. Historically, with firearms of the 18th century, shooters had to assemble  these components manually in order to reload, inserting black gunpowder,  wadding,  and  a  spherical  lead  ball  into  the  gun’s  barrel.  With  the  intent  of making reloading faster, early cartridges featured premeasured and pre- packaged charges of powder, in small bags, but they still required an exter- nal source to provide a thermal “flash” to ignite the powder and fire the  projectile. The innovation of the breechloader, by which the ammunition  is loaded at the rear of the gun’s barrel, made modern integrated ammuni- tion possible. Modern ammunition links these three components together,  placing them inside an outer case. Ammunition is commonly identified based on the diameter of its bullet,  for proper fitting with firearms barrels. The original designation of ammu-

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 BALLISTIC IMAGING nition size was by caliber: The unit of measurement was hundredths of an  inch (e.g., .38 caliber corresponding to a bullet with diameter 0.38 inches).  However,  such  caliber  labels  are  only  approximations,  for  example,  a  .38 caliber is actually 0.357 inches in diameter and a .40 caliber is actually  0.429 inches in diameter. Ammunition (and corresponding gun barrels) are  also now identified using the metric system, such as 9mm or 10mm.2  Ammunition  cartridges  are  primarily  divided  into  two  categories— rimfire and centerfire—depending on where the primer is located (and, cor- respondingly, where the gun’s firing pin strikes the cartridge during firing).  We explain the distinction in the next section. 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.  “Lead  styphnate  is  the  main  ingredient,”  generally,  although  individual primers may also include some of the following: “[trinitrotoluene  (TNT)], lead or copper sulphocyanide, lead peroxide, sulfur, tetryl, barium  peroxide, and barium nitrate” (Rinker, 2004:19). Ground glass may also  be added as a “sensitizer,” to create friction when impacted by the firing  pin (Matty, 1987:10). A primer mixture is a high explosive; working with  it  and  placing  the  primer  in  the  case  are  extremely  sensitive  parts  of  the  ammunition manufacture process. Rimfire cartridges were first developed in the 1800s, and rimfire ammu- nition remains in heavy usage in .22 caliber cartridges. As the name implies,  “the primer composition is spun into the rim of the cartridge case,” putting  it in immediate contact with the powder propellant (Rinker, 2004:19–20).  By comparison, centerfire ammunition has a cylindrical cap seated in the  cartridge head that contains the primer mixture. The cap consists of a cup- 2  are  is  needed  with  the  use  of  the  word  “caliber.”  Here,  “caliber”  is  shorthand  for  the  C nominal caliber  of  the  ammunition,  which  refers  specifically  to  the  diameter  of  the  bullet.  However, specific caliber of ammunition “refers to a name given to a cartridge representing  the  entire  design  of  the  cartridge  as  intended  by  the  manufacturer,  [including  not  only]  the  diameter of the bullet but the entire shape and size of the cartridge” (Moran, 2000:235). That  is, a nominal-caliber ammunition group may include a wide variety of specific varieties that  can vary significantly in their length, case design, powder charge, and so forth. Both “nominal  caliber” and “specific caliber” are used to describe and label firearms as well, referring to the  “group of firearms which share the same bore diameter” and the “name given to a firearm  representing  the  specifically  designed  cartridge  which  will  fit  into  the  firearm,”  respectively  (Moran, 2000:235).

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 FIREARMS AND AMMUNITION and-anvil  combination  and  a  pellet  of  primer  mixture.  During  firing,  the  firing pin “compresses the primer composition between the cup and anvil,”  causing a flame that passes through a hole or vent to ignite the propellant  charge  (Rinker,  2004:19).  Practically,  the  development  of  the  centerfire  system “was the great milestone in weapon and ammunition development;”  with it, “only the primer cup needed to be soft enough to be crushed by the  firing pin,” freeing the main body of the cartridge case to be harder, provid- ing “a gas seal for much higher pressures than could be obtained with rim- fire ammunition” (Heard, 1997:11). Centerfire cartridges also developed,  in part, due to the desire to reuse “the most expensive part of the cartridge,  the case”; the centerfire configuration permits new primer assemblies to be  inserted into expended casings (Matty, 1987:8).  Given  its  purpose,  the  primer  assembly  must  meet  specific  criteria.  The primer mixture “must always have a uniform flash that is hot enough  without being too violent. In other words, it must always consistently pro- duce the proper amount of heat” (Rinker, 2004:20). Likewise, the material  holding the primer—either the cartridge brass of the rim in a rimfire car- tridge 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. Centerfire primer cups are typically  brass or nickel. Propellant Though  it  derives  from  centuries  of  development,  a  critical  part  of  ammunition  is  subject  to  popular  misunderstandings  and  mislabelings.  It  is commonly referred to as powder, tracing from ancient formulations of  black  powder  and  more  modern  incarnations  of  smokeless  gunpowder.  As Hatcher (1935:96) observes, powder “originally meant, and still does  mean, fine dust; but at the present time we find substances called powder  which do not in any manner resemble dust and which are not even finely  divided.” Propellant is a more generic and more apt term for the substance  used in modern ammunition. The individual particles of propellant may still  be referred to as grains, even though they may not have a gritty or granular  texture; however, the common use of grains to describe the exact quantity  or  charge  of  propellant  in  a  cartridge  has  nothing  to  do  with  texture  (a  grain is a measured weight equal to 0.0648 grams). Fundamentally,  a  propellant  is  not  devised  to  explode violently:  It  is  designed to burn, and burn rapidly. As Rinker (2004:21) summarizes, “all  gunpowder produces the force to move a projectile as the result of 3 things.  (1)  When  it  burns,  it  produces  a  huge  quantity  of  gas.  (2)  As  it  burns,  it  produces  a  huge  amount  of  heat.  (3)  After  ignition,  it  creates  its  own  oxygen and needs no outside air. All three are required. At first, the need 

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 BALLISTIC IMAGING for heat may not be as obvious as the other two, but hot gas expands and  requires more space then cold gas,” heightening the buildup of pressure in  the gun’s chamber. Modern  propellants  are  a  form  of  nitrocellulose,  first  discovered  in  1846 when cotton, nitric acid, and sulfuric acid were mixed. One pound of  nitrocellulose-based powder contains 1.2–1.5 million foot pounds of stored  chemical energy, in comparison with about 600,000 foot pounds of stored  energy in one pound of the traditional saltpeter, charcoal, and sulfur com- bination of black powder (Rinker, 2004:23). “If ignited in an unconfined  space,” nitrocellulose propellant will burn gently; if, however, combustion  occurs in a confined space—as in a cartridge—“the heat and pressure built  up will accelerate the rate of combustion exponentially” (Heard, 1997:76).  The  charge  of  propellant  utilized  in  cartridges  is  carefully  tuned  to  the  caliber, bullet weight, barrel length, and desired performance of the ammu- nition. Chemical “moderating” agents or other additives (e.g., graphite or  barium nitrate) are often used to control the burn rate of the propellant,  and  the  mixes  used  in  final  propellants  are  “very  tightly-controlled  trade  secrets” (Heard, 1997:59). Cartridge Cases Cartridge  cases  have  traditionally  been  manufactured  from  brass,  an  alloy of copper and zinc, although other materials have been used; in partic- ular, steel casings (coated with copper or a lacquer) were developed during  World War II due to brass shortages, and steel cases remain in use in some  countries because of their lower cost. Cartridge brass is almost universally  of  the  same  composition:  a  70-to-30  or  75-to-25  alloy  (in  percentage  of  weight) of copper and zinc, respectively. This combination was developed,  along with methods for working with it, as a result of the physical demands  put on the case during the firing of a gun. As described below, a cartridge  case expands during firing, pressing against the chamber walls to create a  seal and containing the high-pressure gases created in firing. To accomplish  this in situ deformation, the hardness of the cartridge brass must be precise  so that the case retains its original shape and can be readily extracted from  the breech. Too hard a starting brass and the case may crack during firing;  too soft and it will expand and deform too much and be difficult to extract.  Although there are a number of manufacturing processes currently used to  produce cartridges, the salient features of the general manufacturing process  are  similar.  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.

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9 FIREARMS AND AMMUNITION One  modern  manufacturing  process  for  producing  a  centerfire  case  starts with brass rod or wire, in coils. A machine called a cold header, simi- lar to the one used to make common nails, feeds in the rod or wire, cuts off  a piece large enough to make one case, and transfers it to a cavity in the  machine, where it is struck by a punch. This process forms the irregularly  shaped cylindrical piece into a precise sort of button shape. The button is  annealed (heated and then cooled) to reduce its hardness, and is then fed  into a two-stage transfer press that transforms the cartridge blank into a  low, wide cup. The half-formed cup is next pushed through a die or series  of dies that draw the blank to its final shape and dimensions. Additional  annealing, cleaning, and forming steps are done sequentially until the blank  is in the final shape of the cartridge case.  bullets The last major component of the cartridge  is  the bullet  or  projectile.  Bullets in modern ammunition can consist of a variety of metals. There are  bullets made entirely of aluminum, steel, and sometimes brass; nonmetallic  substances like rubber and wood have also been used to make bullets. How- ever, to provide the needed weight for improved accuracy and performance,  bullets most often contain some amount of lead.  Bullets  are  designed  for  two  basic  purposes—penetration  on  impact  with  a  target  and  perforation  and  expansion  to  increase  damage—and  the  exact  composition  and  construction  of  bullets  are  tailored  to  those  purposes. An all-lead bullet is very soft and therefore expands rapidly on  striking a target. Indeed, “pure lead is not used for lead bullets” precisely  because “it is too soft [and] damages too easily in handling and loading”;  antimony is most commonly added to lead as a hardening agent, though tin  has also been used (Frost, 1990:27). Better penetration power at greater dis- tances and accuracy can be attained by covering a lead core with a full jacket  or  partial  jacket  composed  of  a  copper  alloy.  High-velocity,  fully  jacketed  bullets are designed to penetrate deeply, while lower velocity jacketed bullets  may tumble within the target and cause additional damage due to expansion.  Mushrooming or expanding bullets, such as hollowpoints, are designed to  transfer a maximum amount of energy to the target and to penetrate but not  exit. The composition and design of bullets—along with what materials they  do or do not strike—are important to forensic ballistics analysis as they affect  what condition a recovered bullet will be in and hence how difficult it is to  match to other evidence. A  lubricant  is  applied  to  bullets  before  they  are  seated  in  cartridge  c   asings;  it  acts  to  cut  down  on  metal  fouling  of  the  bore,  the  deposition  of particles or residues from the bullet (Frost, 1991:31). In centerfire car- tridges,  where  “grease  grooves”  are  created  in  the  case  by  knurling,  the 

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0 BALLISTIC IMAGING lubricant  is  usually  a  wax  or  heavy  grease  type;  due  to  its  placement,  it  must  be  a  substance  that  will  neither  contaminate  the  powder  nor  react  with lead or copper plating.  2–b THE FIRINg OF A WEAPON: INTERNAL bALLISTICS The general concept of “ballistics” can be divided into separate stages;  see Box 1-1. External ballistics (the flight path and behavior of the bullet  between its exit from the barrel and its arrival at its target) and terminal  ballistics  (behavior  of  the  bullet  on  striking  a  target)  are  both  critical  to  complete firearms investigations.  Our  primary  focus  is  on  internal  ballistics—the  actions  that  occur  between the pulling of the trigger and the bullet’s exit from the barrel of  a firearm. Internal ballistics is “a series of actions or operations that every  firearm  must  go  through,  whether  .22  caliber  revolver  or  a  .50  caliber  machine gun,” all of which occur in a time span on the order of 0.003 sec- onds  (Rinker,  2004:1,  2).  The  trigger  pull  starts  the  mechanical  process  of allowing the firing pin to strike the primer of the chambered cartridge.  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. “When the pressure has built up to a sufficient level, known  as short shot, the bullet will start to move because the pressure is greater  than the holding force of the case neck.” As the powder burn continues,  “the pressure increases and the neck and body walls of the case expand to  meet and grasp the inside chamber walls,” creating a seal and increasing the  pressure acting on the bullet’s base, propelling it forward (Rinker, 2004:1).  The bullet, being slightly larger than the barrel diameter, is forced to seat  into the rifling (the lands and grooves) on the bore of the barrel, picking  up rotation as it passes down the length of the barrel.  While this sequence of events drives the bullet through the barrel and  out  of  the  firearm,  forces  are  also  at  work  on  the  head  of  the  cartridge.  Hatcher (1935:270, 272) describes the processes for a centerfire cartridge: When a primer is struck by the firing pin, the very brusque and powerful  mixture  that  it  contains  explodes  with  violence,  [causing  the  flame  that  ignites the powder charge]. But the explosion of the primer mixture also  reacts  in  a  backward  direction  onto  the  primer  cup  itself,  and  blows  it  part way out of the primer pocket, unless the primer is strongly crimped  in place, as is done with some kinds of rifle ammunition. Then when the  main  charge  ignites,  the  powder  pressure  inside  the  case  forces  the  case 

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 BALLISTIC IMAGING Ammunition Brand Remington-Peters Speer Wolf Glock Firearm Brand S&W FIguRE 2-2  Breech face markings and firing pin impressions for three ammunition  2-2.eps types and two firearm brands. NOTE: S & W = Smith & Wesson. SOURCE: Adapted from Tulleners (2001:Fig. 3-4). cartridge presses when it is being fired. These marks are quite pronounced  on metal surfaces that have been finished by a file as is commonly done on  the breech face of the average [semi]automatic pistol or revolver. Examined  under a microscope this surface appears to consist of a number of ridges  or scratches, and when the cartridge is fired, the primer, being of copper  or brass, which is much softer than the steel of the breech face, will take  the impression of these fine ridges. In  gross  appearance,  features  in  the  breech  face  impression  may  fall  into  some  general  categories  depending  on  the  specific  filing  or  polishing  steps used by the manufacturer. Straight filing creates linear features; other  breech  face  impressions  may  feature  cross-hatching  or  circular  patterns.  For example, Kennington (1995) 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 

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 FIREARMS AND AMMUNITION may also be evident elsewhere on the cartridge head. Kennington suggests  that the rifling characteristics from bullet evidence at a crime scene can be  combined with evidence of arched markings on cartridge casings to rapidly  identify the pistol make in question.3 Because breech face impressions are created by the pressure of firing,  Tulleners (2001:3-2) notes that their detail “is dependent on cartridge cham- ber pressure and the type of breech face manufacture/condition. [Chamber  pressure  varies  within  caliber  and  depends  on  such  factors  as  the  bullet  size and weight and the powder charge contained in the cartridge.] 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).  Firing Pin Impressions The firing pin impression on the surface of the primer provides impor- tant information on the general class of the firearm that discharged a casing.  The shape of the “pit” marking the firing pin’s strike indicates the shape of  the firing pin in the firearm (e.g., round, elliptical, rectangular). The firing  pin impression will also bear the marks created by filing or smoothing the  tip of the firing pin. “The point of the firing pin will have small ridges, and  no  two  .  .  .  firing  pin  points  will  be  exactly  alike,”  conjectured  Hatcher  (1935:266).  However,  Burrard  (1962:113)  notes  that  “great  caution  is  necessary” in distinguishing individual markings from grosser features of  firing pin marks, which “often take the form of a number of small concen- tric rings.” Yet individual imperfections on the tip of the firing pin can be  telltale: “Another by no means rare feature of a [firing pin] is the presence  of a  small ‘pimple’ on  the  extreme end,” and so the presence  of a corre- sponding  mark  on  one  cartridge  and  the  absence  on  another  “would  be  proof positive that the [second] cartridge could not have been fired” from  the same weapon as the first. For some guns and some firings, the firing pin impression may not be  a clearly defined indentation on an otherwise flat surface. Instead, primer  “flowback”  may  occur:  a  larger  crater  is  created  as  the  primer  material  3  owever, he cautions that “the arch-producing machine process . . . may not be the final  H breechface treatment at the factory. The breechface can still be broached, filed, sandblasted,  tumbled  and/or  plated,”  and  residue  buildup  as  a  result  of  firing  can  obscure  the  arch  markings.

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 BALLISTIC IMAGING around the pit is forced outward by gas pressure, partially flowing into the  aperture in the breech from which the firing pin emerges. Though “flow- back” is commonly attributed to firearms in which excessive pressure can  build  during  firing,  Kreiser  (1995)  suggests  other  explanations  that  also  correspond  to  characteristics  of  the  particular  make  of  firearm.  Among  these is the diameter of the firing pin aperture: the wider the aperture, the  more primer surface is unsupported (not positioned directly against another  object) during firing and hence more likely to crater outward. In  some  firings,  the  firing  pin  may  scrape  against  the  surface  of  the  primer as it is withdrawn. In these cases, the firing pin impression is not  purely a mirror of the shape of the firing pin (e.g., circular) but has a drag  mark  trailing  away  from  the  main  impression.  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 fir- ing pin impacts the cartridge across different firings; the pin may wobble  slightly and strike at slightly different points and angles.4 In rimfire weapons, the firing pin strikes the brass of the outer rim of  the cartridge head. As Hatcher (1935:68) observed, “[rimfire ammunition]  takes  a  good  impression  showing  the  shape  of  the  firing  pin,  but  it  does  not often take a clear impression of the fine file marks and other irregular  scratches on the breech block, which form the ‘finger-prints’ of the gun.”  Accordingly, he noted that “when an empty rim fire cartridge is found at the  scene of a shooting, it is often easy to say what type of arm was used; but  it is seldom possible to identify a rimfire cartridge to a definite individual  gun by the impression of the file marks it left on the head, as is so often  done in the case of a center-fire cartridge.” Ejector Marks The ejector arms in automatic or semiautomatic firearms can vary in  shape  (e.g.,  rectangular,  round,  or  triangular)  and  size;  the  footprint  of  the  ejector  determines  the  size  and  shape  of  the  mark  left  by  the  ejector  on the rim of the spent casing. Ejector marks can vary from tiny divots to  4  adal (1995) provides an unusual but vivid example of the difference that placement and  F angle of the firing pin strike can have on the resulting marks. The Hi-Standard Model DM-101  is a .22 caliber derringer handgun that is double-barreled; however, the same rectangular firing  pin is used to initiate the firing in each of the two barrels. The difference in the way the same  pin  hits  the  (rimfire)  casings  in  the  two  barrels—one  using  the  top  part  of  the  pin  and  the  other the lower—is sufficiently large that an examiner cannot match firings from one barrel  to firings from the second barrel on the firing pin marks alone.

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 FIREARMS AND AMMUNITION more  substantial  indents  on  the  cartridge  head  near  the  rim.  Analysis  of  ejector marks can be made more difficult by the fact that the rim of the car- tridge head is also where ammunition makers put their headstamp (brand  identifier) and information on the size and caliber of the cartridge. These  heavy-set  alphanumeric  characters  are  inscribed  on  the  cartridge  brass  and—depending on where the ejector happens to hit—parts of the stamp  may bleed into the ejector mark. 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 orienta- tion 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; particu- larly in semiautomatic weapons, other firearms parts are used to circulate  ammunition through the weapon and eject spent casings. These actions and  parts can lead to a host of marks on the cartridge case that—though not  imaged using current techniques—are sometimes used by examiners study- ing matches between pieces of evidence. Chamber  marks  are  parallel  striated  marks  along  the  outer  walls  of  the cartridge case, impressions from the scraping used to bore or ream the  chamber (along with the rest of the barrel) from a solid piece of alloy. The  extractor in a pistol that helps move a spent cartridge out of the chamber  is  typically  a  small  arm  that  fits  over  the  rim  of  the  casing,  holding  it  as  the breech assembly slides backward. Accordingly, the extractor can leave  marks where it makes contact, either on the edge of the rim of the cartridge  head or on the neck separating the head from the main body. The slide that  moves back and forth in semiautomatic pistols, allowing ejected casings to  move away from the weapon, may leave a scuff mark on the edge of the  cartridge  head  and  a  rough  drag  mark  along  the  cartridge  wall.  As  indi- vidual cartridges move from a magazine into chamber, a mark on the outer  wall of the case may be caused by the magazine lip. 2–C.2 bullet Markings Hatcher’s  (1935:255)  seminal  text  on  firearms  identification  referred  to “the fine ridges and grooves on the surface of the bullet, parallel to the  rifling  marks,”  as  “the  most  important  individual  characteristics  which  are used” in the field. These marks on the bullet—known as striations or  striae—“are  caused  by  its  passage  over  surface  irregularities  and  rough  spots  on  the  interior  of  the  gun  barrel  that  got  there  principally  during 

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 BALLISTIC IMAGING the machining operations of reaming the bore and rifling the grooves. Any  such  machining  operation  will  leave  the  bore  at  least  slightly  rough,  and  each rough spot will leave a mark on the bullet during its passage through  the bore.” The rifling carved into the barrel takes the form of grooves separated by  raised areas, known as lands. These lands and grooves create corresponding  engraved  areas—dubbed  land  engraved  areas  and  groove  engraved  areas  (and  commonly  abbreviated  as  LEAs  and  GEAs)—on  the  bullet  surface,  separated  by  shoulders.  The  land  engraved  areas,  being  the  part  of  the  b   ullets that scrape against the raised lands on the barrel, are the principal  areas of interest for observing striations. The pattern of land and groove engraved areas on recovered bullets can  be used to determine basic information about the rifling characteristics of  the gun that fired them, in order to identify a class of guns from which it  came. Specifically, the number of lands is an important class characteristic,  as is the direction of twist evident from a side  view of  the  bullet. Bullets  (and corresponding rifling characteristics) are commonly labeled by these  two pieces of information—e.g., 5R for five lands and a right-hand twist. A  recovered bullet can also be measured to suggest the caliber of the ammu- nition  and  weapon.  However,  this  is  not  always  possible—nor  is  a  full  analysis of striation marks—due to the condition of some bullets recovered  from crime scenes (and victims). Bullets  fired  through  weapons  using  polygonal  rifling  create  special  difficulties.  Compared  to  conventional,  square-edged  rifling,  polygonal  rifling has key advantages: it reduces metal fouling, and it increases bullet  v   elocity by reducing friction as the bullet passes through the barrel (Heard,  1997:123). However, the smoothness and subtlety of polygonal rifling can  make  it  difficult  to  discern  even  gross  features  on  recovered  bullets—the  shoulders  defining  lands  and  grooves—much  less  fine  individual  detail.  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)  ways.  Manufacturing  processes  are  also  essential  to  consider  in  assessing  the  costs  and  benefits  of  wide-scale  ballistic  imaging  or  alternatives  such  as  microstamping. Introducing stages to the process of producing firearms or  ammunition—for example, systematic test-firing to produce exhibit cases,  imaging of exhibits in large batches, or laser-etching a unique mark on the 

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 FIREARMS AND AMMUNITION base of a bullet—can have major impacts on the cost of production and,  perhaps, the feasibility of compliance with proposed changes. We have already touched on some aspects of manufacturing in describ- ing  the  anatomy  of  firearms  and  ammunition  earlier  in  this  chapter,  and  aspects of manufacture will arise in Chapter 3 as well (particularly in dis- cussing challenging issues for firearms identification, generally). This section  introduces basic issues but is not a comprehensive discussion. 2–D.1 Firearms The manufacturing of most guns is highly automated and generally effi- cient, and as many as 5 million new firearms (domestic and foreign) enter  the  U.S.  market  each  year.  Befitting  its  historical  development,  dating  to  Samuel Colt’s popularization of interchangeable parts and production line  assemblies, the modern firearms industry remains one that is characterized  by solid process control. 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 machin- ing and assembling firearms, and choices on the amount of filing or polish- ing to do on firing pins or whether to apply paint to the breech face can  have  an  impact  on  the  resulting  toolmarks.  In  addition,  some  manufac- turing  techniques  affect  the  type  and  quality  of  marks  created  in  firing.  Champod et al. (2003:307) argue that “machining marks made by grinding,  filing and some other machining methods are random and hence we expect  no repeatability between tools.” In comparison, “machining marks made  by stamping, some cutting processes such as broaching, and some forging  processes may be repeatable.”  Various  manufacturing  techniques  used  by  Lorcin  Engineering  drew  interest in the 1990s, as firearms produced by the firm became more widely  used  in  crimes;5  they  serve  as  useful  illustrative  examples.  Thompson  (1996:95) found two Lorcin L9MM semiautomatic pistols, bought at the  same time, that produced sufficiently similar breech face markings that a  match  could  be  made  to  either  weapon  on  that  mark  alone;  they  could,  however, be distinguished by sidewall and extractor marks. Similarly, Matty  5  n 2000, the Lorcin L380 semiautomatic pistol was the most traced firearm after recovery  I from juvenile possessors, and a Lorcin .25 caliber pistol ranked seventh. The L380 was also  traced with high frequency after recovery from older offenders, ranked second among firearms  recovered from 18–24-year-olds, and ranked third among firearms recovered from adults aged  25 and older (U.S. Bureau of Alcohol, Tobacco, and Firearms, 2002:15–16).

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 BALLISTIC IMAGING (1999:134) reports on a case where a search on a DRUGFIRE database—an  initial competitor to the current Integrated Ballistics Identification System  (IBIS)  for  ballistic  imaging  (described  in  Chapter  4)—suggested  enough  similarity  to  cause  the  physical  evidence  (both  test-fired  cartridge  casings  and the recovered Lorcin L9MM that produced them) to be retrieved from  storage. On more detailed examination, “the breech face signatures were  similar,  but  there  was  insufficient  detail  for  an  identification”;  however,  chamber and extractor marks failed to coincide at all.  “The heavy black ‘paint’ that adhered to the breech face” was origi- nally  believed  to  be  a  cause  of  this  phenomenon  (Thompson,  1996:95).6  Ultimately,  though,  it  was  attributed  to  the  fact  that  the  breech  faces  for  that model being formed by stamping, with no further grinding. In earlier  Lorcin models, “the breechface area would become battered during firing  as [a relatively soft alloy slide] hit the rim of a cartridge in the magazine  as it fed the cartridge into the chamber”; this caused the breech face mark- ings to be unstable and to change from firing to firing (Matty, 1999:135).  Lorcin revised its process—in newer models, “a solid stamped steel insert  is  placed  into  a  non-ferrous  alloy  slide”—but  this  stamped  steel  insert  is  prone to have marks that “can carry over from one steel insert to another”  (   Tulleners, 2001:3-4). (This phenomenon is an example of subclass carry- over, discussed in fuller detail in Section 3–B.1.)  More  generally,  Collins  (1997:498)  observed  that  “the  bullets  and  c   asings of the [Lorcin] L380 [.380 caliber semiautomatic] pistol are easy to  characterize.  The  bullets  exhibit  slippage7  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.  Another  example  of  manufacturing  processes  that  can  directly  affect  the marks left by firearms and the ability to match them is the button rifling  technique  used  by  some  manufacturers,  notably  Hi-Point  (Roberge  and  Beauchamp, 2006:166):  6    thick  coat  of  black  paint  was  also  judged  to  be  the  probable  cause  of  highly  similar  A breech face marks produced by two different 45 ACP Haskell semiautomatic pistols; individual  characteristics would emerge on the breech face marks for each gun with repeated firings, as  the paint chipped and wore off (Tulleners, 2001:3-4). 7  Slippage” means that a bullet does not fully grip the rifling on the barrel interior; hence,  “ it can wobble and shift, rather than following the clear path of the rifling (and having marks  carved into the side of the bullet as it passes through).

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9 FIREARMS AND AMMUNITION This process creates the grooves in the barrel by compressing rather than  removing the excess material resulting in a relatively shallow barrel groove.  Another distinct characteristic of the Hi-Point barrels is the metal tailings  left along the shoulder of the groove. The combination of button rifling  and  metal  tailings  creates  a  relatively  smooth  barrel  with  very  coarse  shoulders.  With  each  shot  fired,  all  or  part  of  the  metal  tailings  break  off changing the coarse stria on the fired bullet. The shallow rifling also    llows  a  great  deal  of  slippage  to  occur.  Furthermore,  the  crowning8  of  a these barrels can add additional subclass characteristics.  All newly manufactured firearms are required to bear a unique serial  number, and this number may be stamped or etched on various parts of the  firearm frame and assembly. However, guns with consecutive serial numbers  are generally not consecutively manufactured in full. Production of firearms  is typically an assembly line process, drawing various preconstructed parts  from  large  bins  for  assembly  into  a  finished  weapon.  Hence,  two  fire- arms that bear consecutive serial numbers may have rolled off the line in  sequence, but their frames, barrels, firing pins, and so forth need not have  been manufactured right after each other. There are some exceptions to this  rule; for instance, Lardizabal (1995) found that consecutive serial numbers  in a set of Hechler & Koch 9mm USP semiautomatic pistols meant that the  slide for these weapons had in fact been consecutively manufactured.9 2–D.2 Ammunition Like firearms, ammunition cartridges are the result of numerous tool- ing  and  machining  operations,  and  individual  manufacturers  vary  in  the  specific  techniques  they  use.  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  post- processing steps, such as the application of a lacquer sealant to the primer  surface.  “Primer  sealants  are  routinely  applied  to  centerfire  cartridges  to  increase the power and reliability of the ammunition,” “placed at the junc- tion between the primer and the primer cup [to] create a water and airtight  8  “Crowning”  is  a  finishing  step  on  the  muzzle  or  discharge  end  of  a  barrel,  rounding  or  grinding the mouth so that it is flush or recessed slightly and thus providing no obstacle to  the bullet’s exit. 9  Lardizabal  (1995:50)  found  that  firings  from  a  set  of  these  pistols  with  similar  serial  n   umbers could not be  distinguished  from  each  other  by  any  mark,  and  this  “persistence  of  detail”  continued  through  250  firings.  A  pattern  of  striations  was  observed  on  the  breech  face itself, above the firing pin hole; this mark appeared to have been created after a chemical  finishing process.

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0 BALLISTIC IMAGING seal [and prevent] oil and other foreign matter from entering the cartridge.”  The sealant also makes the cartridge resistant to moisture. However, while  “most ammunition manufacturers limit the application of the sealant to the  junction of the primer and primer cup,” some (primarily European) manu- facturers “apply the sealant so that it extends across the entire surface of  the primer.” The Czech-made Sellier and Bellot ammunition, in particular,  is  known  for  a  red  lacquer  sealant  over  the  entire  primer  (Hayes  et  al.,  2004:139). The lacquer can act as a cushion, “absorb[ing] and dissipat[ing]  a greater amount of energy” when involved in a collision (compared with  metals), and consequently “reduc[ing] the amount  of energy that reaches  the metal surface of the primer” (Hayes et al., 2004:142). The  specific  techniques  of  a  manufacturer  can  combine  with  more  ornamental and postprocessing steps to leave distinctive marks on the car- tridge. Box 2-1 reviews these nonfiring manufacturing marks—features that  are present on the cartridge before firing and traces of which may endure  after  firing.  In  comparing  exhibits,  firearms  examiners  must  compensate  for the presence of these nonfiring marks, lest they lead to a false identi- fication or exclusion. While many of these nonfiring marks are deliberate  design choices, others arise inadvertently due to other steps in manufacture.  Yborra and McClary (2004) 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.  Remington  managers  indicated that they had never previously experienced such a phenomenon  but suggested that a possible cause might be the way the primer is seated  in  the  cartridge.  Two  separate  punches  drive  the  primer  to  its  final  posi- tion about 0.002 to 0.005 inches below the level of the cartridge head; “a  misalignment  or  damage  to  one  of  these  punches  MAY  have  caused  the  observed [marks], and being machine-based, would be consistent” (Yborra  and  McClary,  2004:309).  But  no  such  defect  could  be  found;  nor  could  similar marks be detected on other boxes of ammunition from the same lot.  The punches used in primer seating were also suspected of causing parallel  markings near the edge of the primer on some Winchester 9mm ammuni- tion (Flater, 2002:315); it was also suggested that the die used to flatten the  surface of the primer cup could also have impressed such a mark.

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 FIREARMS AND AMMUNITION BOX 2-1 Nonfiring Manufacturing Marks Nonfiring manufacturing marks on ammunition are features created by indi- vidual firms’ manufacturing processes. They are not defects, in that they do not diminish the ammunition’s performance or otherwise detract from the ammuni- tion’s quality. However, they may be mistaken for textures or striations created by the firing of a gun or that may complicate the determination of a pattern match between exhibits. Amassing knowledge of these marks—and developing the skill to adjust for their presence—is an important part of the experience of a firearms examiner. Cataloging these nonfiring manufacturing marks, Tam (2001) suggests a rough typology based on their impact on the determination of a match between evidence: (1) marks that are not expected to cause a problem for identification (or exclusion); (2) marks that may cause problems but can be compensated for with some effort; and (3) marks that are problematic for comparison and difficult to analyze. In the first class, there are marks that would easily be overwritten by firing- related marks, as in extremely fine pre-existing parallel marks on the primer surface. Other marks—being relatively simple and known in advance—are not problematic because the examiner can mentally compensate for their presence (e.g., a V-shaped or other stamped mark on the primer surface used to indicate certain brands). Other marks that may fall into this category are those that are on areas of the cartridge not typically considered for ballistic imaging or routine analysis, such as unique marks on the rim of the cartridge. For the second class, manufacturing marks that may cause problems, Tam (2001) suggests that these features can be overcome by simple procedures. 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. An IBIS image (using side light) of a fired round of Wolf ammunition is shown below; most of the visible horizontal parallel marks on the primer surface existed prior to firing. Side light IBIS image of fixed casing using Wolf ammunition; heavy hori- zontal lines are preexisting manufac- turing marks. continued

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 BALLISTIC IMAGING BOX 2-1 Continued Reitz (1975:103) observed “matchable striations on unfired primers of [exhibits from particular lots of] Winchester, .38 special cartridges.” These marks were attributed to a particular punch used during the primer seating process, which had not been produced to the same smoothness as is typically the norm. “These mark- ings remained prevalent even after firing, which could be perilous to comparison examinations by unwary examiners.” Similarly, Robinson (1996:164) 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) reports on toolmarks on the primer surface of Fiocchi .25 Auto ammunition whose cause is unknown; the manufacturer suggested that they might be attributed to a rare, imperfect configuration of the feeder during the process in which the primer is seated in the empty shell. The marks were problematic because they were not consistently prominent across the whole primer surface. When, as in the Wolf ammunition toolmarks, the markings span the whole primer, an examiner can compensate for them because they can be traced from the face of the primer into the pit of the firing pin impression. Maruoka (1994a; see also Maruoka and Ball, 1995) had previously noted parallel marks on the primer sur- face of some Fiocchi ammunition, but those marks did span the entire surface. But these inconsistent marks offer no such traceability, so that “differentiating these marks from breech face marks would be very difficult, if not impossible” (Murray, 2004:314). Some ammunition may also bear random marks on the rim of the cartridge that could be mistaken for ejector marks.