Introduction
Firearms are heat engines which convert the chemical energy of a propellant into kinetic energy of a projectile weapon. A firearmre quires only three components: a projectile, a barrel to confine the hot gases generated by the combustion of the propellant and to guide the projectile toward its target and a source of ignition for the propellant. Since the first appearance of handguns in the fourteenth century a variety of ignition systems have been used: matchlocks, wheel locks, flintlocks and finally percussion locks. Methods of loading firearms have changed from muzzle loading to breech loading. The accuracy of firearms has also been improved by the introduction of rifling, which imparts a stabilizing rotation to the projectile.
Modern Firearms
The variety of modern firearms can be understood by considering the functions that must be performed in order to fire metallic cartridge ammunition. First a round of ammunition must be loaded in the firing chamber of the weapon. The firing mechanism must be cocked, rendering the weapon ready to fire. Squeezing the trigger actuates the firing mechanism, discharging the round of ammunition. The expended cartridge must be extracted from the firing chamber and then ejected from the firearm so that a fresh round of ammunition may be loaded. In earlier firearms all or most of these functions were carried out manually by the shooter. As firearms designs have progressed more and more of these processes have been rendered automatic with the energy they require being obtained from the firing of the cartridge.
The last decades of the nineteenth century saw a revolution in the development of automatic and semiautomatic (or self-loading) firearms. In these weapons some of the energy released when a cartridge is fired is used to extract and eject the spent cartridge, cock the firing mechanism and load a fresh round of ammunition from a belt or from a magazine. They accomplish this in one of three ways. In the ‘locked-breech’ system, the weapon’s barrel and bolt are locked together and recoil a short distance to the rear as a unit when a cartridge is fired. The barrel is then cammed away from the bolt and the bolt continues to the rear, extracting and ejecting the spent cartridge. The firing mechanism is also cocked by this rearward motion of the bolt. The recoil energy of the bolt is stored (usually in a recoil spring) and is fed back into the bolt. The energy supplied by the recoil spring causes the bolt to move forward, loading a fresh round into the weapon’s chamber. Hiram Maxim’s machine gun designs (1882-1885) were the first to exploit recoil operation. In 1888 Salvator and Dormus patented the ‘blowback’ system. In this system the barrel and breechblock are not locked together; the breech is held closed by the pressure of a heavy spring. When a round is fired the inertia of the breechblock and the pressure of the spring hold the breech closed long enough for the bullet to exit the barrel. Then the pressure in the firing chamber forces the breechblock to the rear, compressing the spring. The rearward movement of the breechblock extracts and ejects the cartridge and cocks the firing mechanism. The compressed spring pushes the breechblock forward again, loading a fresh round into the firing chamber. Blowback weapons are chambered for pistol cartridges (e.g. 9 mm parabellum); they cannot fire the powerful cartridges used in rifles. In 1890 John Moses Browning demonstrated the gas-piston system. In gas-operated firearms a small amount of the pro-pellant gases is allowed to escape from a hole in the barrel into a gas piston. The piston is linked to the breechblock by an operating rod; as the propellant gases force the piston to the rear the breechblock is also forced to the rear against a heavy spring. The rearward motion of the breechblock extracts and ejects the spent cartridge and cocks the firing mechanism. The spring forces the breechblock to close and as the breechblock returns to the closed position it loads a fresh cartridge into the firing chamber.
The different types of modern firearms that may be encountered by a firearms examiner are briefly discussed below.
Pistols
These are firearms designed to be held, aimed and fired with one hand (although good pistol marksmanship requires a two-handed grip).
Single-shot and double-barreled pistols Single-shot pistols are usually target pistols. Double-barreled pistols are typically small and easily concealed self-defense weapons.
Revolvers In revolvers, rounds are loaded into firing chambers in a revolving cylinder. The revolver’s firing mechanism rotates the cylinder to bring each chamber under the weapon’s hammer so that the round in the chamber can be fired. Modern revolvers have between five and eight firing chambers in the cylinder. Loading and unloading is manual, using one of three systems: a loading gate through which each chamber must be individually loaded or unloaded; a swing-out cylinder, whose chambers can be simultaneously emptied using a star-shaped ejector; or a break-top frame which is hinged in front of the cylinder.
Self-loading or semiautomaticpistols Modern semiautomatic pistols operate on either the recoil or blow-back system. Magazine capacities vary from six rounds to 33 rounds.
Automaticpistols Some truly automatic pistols have been manufactured, e.g. the ‘broomhandled’ Mauser (either imitations of the M1912 Mauser produced in Spain or the M1932 Mauser Schnell-feuerpistole), the Beretta M951R, the Glock Model 18 and the Heckler and Koch VP70M. Other pistols designated as ‘automatic’ are actually self-loading or semiautomatic pistols.
Machine pistols These are actually submachine guns developed in Germany prior to and during World War II, such as the MP38 and the MP40.
Rifles
These are weapons with rifled barrels designed to be fired from the shoulder. Carbines are short-barreled rifles.
Single-shot rifles These are antiques, replicas of antique weapons or target rifles.
Magazine repeaters
Lever action rifles Manipulation of a lever below the breech extracts, ejects, cocks and loads the weapon. This system was developed in the nineteenth century and appeared in the Henry and Winchester repeaters. The system is still popular and a number of sporting rifles use it.
Slide-action or pump-action rifles Pulling a slide to the rear performs extraction, ejection, cocking and loading. This system has been used on some sporting rifles, but has been more popular for sporting shotguns.
Bolt-action rifles There are two types of bolt actions: turn-bolts (which require the lifting of a handle and rotation of the bolt to unlock it from the chamber) and straight-pull bolts (which require the handle be drawn straight to the rear without turning to open the bolt). The Mauser turn-bolt system is the most widely used bolt action.
Self-loading or semiautomatic rifles The Ml Garand military rifle, which operates on the gas-piston principle, was the first semiautomatic rifle to be adopted as the standard infantry weapon by a major military power. There are currently a large number of military and sporting semiautomatic rifles available.
Automatic rifles Automatic rifles were developed during World War I to provide advancing infantry close supporting automatic fire, contemporary machine guns being too large and heavy to be moved forward rapidly. The Browning automatic rifle (BAR) is an example of this type of weapon. Because automatic rifles were heavy and had magazines with limited capacities they were eventually replaced by light machine guns and assault rifles.
Assault rifles These are light automatic weapons which fire shortened or reduced charge rifle cartridges from magazines. Many have selectors which permit switching between automatic and semiautomatic modes of fire. The first assault rifles (or Sturmgewebr) were developed to give advancing infantry greater firepower. Because they fire rifle cartridges their rounds have greater striking power then submachine guns. Examples of assault rifles include the German MP43 (the original Sturmgewebr) and the Kalashni-kov AK-47.
Submachine guns
These are lightweight automatic weapons; they fire pistol ammunition from magazines. Semiautomatic versions of some submachine guns such as the UZI and the Sterling are also marketed.
Machine guns
These are automatic weapons in which the ammunition is fed from a magazine or a belt. Because machine guns fire more powerful cartridges than submachine guns or assault rifles, they are usually fired from a support such as a bipod or tripod or are mounted in a vehicle such as a tank, an armored personnel carrier or an airplane. Infantry machine guns are usually designed to permit rapid changes of barrel (to avoid overheating and excessive wear of the barrel during prolonged firing); machine gun squads usually carry spare barrels and frequently change them during fire-fights. A number of modern armies use machine gun designs based on the German MG42 general-purpose machine gun; the US M60 general purpose machine gun combines features of the MG42 and the German FG42 automatic rifle.
Shotguns
These are weapons with smoothbore barrels designed to be fired from the shoulder. Their ammunition typically contains small pellets.
Single-shot shotguns This type of shotgun is not as popular in the United States as double-barrel, slide-action or semiautomatic shotguns; they are however quite common in Europe and the Third World.
Double-barreled shotguns The two barrels are either side by side or one above the other (over-under shotguns). Double-barrel shotguns usually break open at the top for loading and reloading.
Magazine repeaters
Lever-action sbotgun This is an uncommon type of shotgun.
Slide-action or pump action sbotgun This is one of the more popular types of shotgun.
Bolt action-action sbotgun This is an uncommon type of shotgun.
Self-loading or semiautomatic sbotgun This type of shotgun has become one of the most popular types.
Improvised weapons
Firearms can be improvised from a variety of components. ‘Zip gun’ is a term applied to firearms improvised from such unpromising material as automobile aerials (for barrels), wooden blocks (for grips), nails (for firing pins) and rubber bands (for main springs). Childrens cap pistols have been modified to fire metallic cartridges as have blank starter pistols. Pocket teargas pins have also been modified to fire cartridges.
Manufacturing Methods
Because firearms examiners must interpret the markings made on fired bullets and cartridges by various parts of a firearm these forensic scientists should have an understanding of the methods used to manufacture firearms and ammunition. Bullets are marked by the rifling in the weapon’s barrel. Conventional rifling consists of lands and grooves that impart a stabilizing rotation to the fired bullet. There are five processes that have been used to rifle gun barrels.
There are two cut-rifling methods. The hook and scrape cutters are the oldest methods for rifling barrels and may still be used by hobbyists. The barrel is prepared for rifling by drilling a hole through the center of a piece of steel bar stock. A hook cutter or scrape cutter is then used to cut the grooves to the desired depth. The hook cutter is an oblong piece of metal with a hook-shaped cutting edge. A metal rod is used to draw the hook cutter through the barrel, shaving away the metal in a groove until the desired depth is reached. As the hook cutter is drawn through the barrel is rotated so that the groove spirals down the barrel. The hook cutter cuts one groove at a time. The scrape cutter functions in a similar fashion; however, the cutting surface of the scrape cutter is a raised ridge. The scrape cutter can have a cutting surface on both sides so that two grooves on opposite sides of the barrel can be cut simultaneously.
The gang broach consists of a series of cutting tools mounted on a metal shaft. Each tool has cutting surfaces corresponding to each of the grooves. The barrel is prepared for rifling by drilling out a piece of steel bar stock. The gang broach is then forced by hydraulic pressure through the barrel, each cutting tool in succession cutting the grooves a little deeper until the desired groove depth is reached. All the grooves in the rifling are cut with a single pass of the gang broach.
Three other rifling methods are also used. In swaging a rifling button composed of tungsten carbide is forced down the drilled-out barrel, simultaneously engraving the lands and grooves of the rifling on the inside of the barrel and expanding the bore to its finished diameter. The metal on the interior surface of the barrel is hardened by cold-working. Inexpensive smallbore firearms are produced by swaging.
In hammer forging, a piece of steel bar stock is drilled out to a diameter greater than the intended finished bore diameter. The drilled stock is then slipped over a hardened metal mandrel and hammered down. The mandrel is then forced out of the rifled barrel. Hammer forging is used to manufacture the barrels of Glock pistols. These pistols do not have conventional land-and-groove rifling; instead the interiors of their barrels have hexagonal cross-sections.
During the process of electrochemical etching, resin layers are painted on to the interior of the drilled out barrel, corresponding to the desired configuration of lands. A chemical solution is then poured into the barrel and used to etch the grooves electrochemically into the interior of the barrel.
Cut-rifling methods produce barrels that have unique microscopic imperfections that produce unique striation patterns on fired bullets - even when the weapon is new. In the case of hook and scrape cutting and broaching the rifling tools wear just enough in rifling a barrel that barrels rifled in succession with the same rifling tools leave distinguishable markings on fired bullets. Swaging and electrochemical etching also appear to produce unique microscopic features within the gun bore. Use of the firearm creates new imperfections in the barrel that mark the fired bullets. These are caused by erosion and corrosion of the gun bore. Erosion refers to the wearing away of the metal of the barrel by friction; corrosion refers to the chemical attack on the metal comprising the barrel by propellant residues. The gradual change in the surface imperfections of the gun barrel can result in a failure to match a questioned bullet to the weapon that fired it, particularly if the weapon has been extensively used between the time the questioned bullet was fired in the commission of the crime and the time it was seized by police as evidence.
Other firearm components of particular interest to firearms examiners are the firing pin, the breechblock or backing plate, the extractor and the ejector. The firing pin strikes the percussion cap of center fire cartridges or the rim of rimfire cartridges in order to detonate the primer compound in the cartridge. Firing pins are finished by hand-filing or by turning on a lathe (a process that also involves hand-filing). These hand finishing operations impart unique microscopic features to the firing pin surface; moreover, firing pins are prone to acquire cracks and other damage from use and abuse. Firearms other than revolvers have a breechblock against which the base of the cartridge rests prior to firing. Upon firing the cartridge is forced back against the breechblock; the primer cap in center fire cartridges and sometimes even the cartridge casing itself will be marked by the surface of the breechblock. Breechblocks are hand filed, milled, turned on a lathe, cut with a broach or stamped. Most of these processes produce a unique pattern of marks which is often referred to as the boltface signature. In the case of revolvers the portion of the frame immediately behind the top chamber in the cylinder (called the backing plate) may similarly mark fired cartridges.
Firing pin marks and breechblock/backing plate markings generally have the greatest significance for firearms examiners because they can only be produced by firing a cartridge in the weapon. Other markings such as extractor and ejector markings may be produced when a cartridge is merely loaded into a weapon and then extracted and ejected manually. In automatic and semiautomatic weapons the extractor is a hook that engages the base of the cartridge and pulls the expended cartridge from the weapon’s firing chamber. Extractors are necessary because firing causes cartridges to expand in the chamber and bind against its walls. Ejectors are usually projections in automatic or semiautomatic firearms that wrest the fired cartridge from the extractor so that it is ejected from the weapon. The term ejector is also used to refer to the devices that are used to remove fired cartridges from the cylinders of revolvers. They take the form either of a rod that pushes the cartridges one at a time from the chamber in the cylinder or a star-shaped piece of flat metal that is used to remove all the cartridges from the cylinder simultaneously. In general, revolver ejectors do not produce forensically useful markings. The ejectors and extractors in semiautomatic and automatic weapons are finished by handfiling.
The Class Characteristics of Firearms
The class characteristics of firearms are those features or attributes that identify the make and model. One task of the firearms examiner is to determine sufficient class characteristics from fired bullets or cartridges to identify the make and model of firearm that fired them. This is not always possible. Most of the time the firearms examiner must be satisfied to limit the makes and models of firearms that could have fired a bullet or cartridge to a small number. The class characteristics determinable from bullets and cartridges are listed below.
General rifling characteristics
Caliber The caliber of a firearm is nominally the bore diameter, i.e. the diameter of a circle that just touches the tops of the lands of the rifling. This diameter may be expressed in hundredths or thousandths of an inch (e.g. 0.38 caliber or 0.380 caliber) or in millimeters (e.g. 5.56 mm or 7.62 mm). It is important to keep in mind that these bore diameters are merely nominal: a weapon designated as 0.38 caliber may have an actual bore diameter that is much smaller. For example, Smith and Wesson revolvers manufactured in a nominal 0.38 caliber have bore diameters ranging from 0.3463 inches to 0.3515 inches. Colt semiautomatic pistols manufactured in a nominal 0.45 caliber have bore diameters ranging from 0.4422 inches to 0.446l inches. The groove diameter (i.e. the diameter of a circle that just touches the bottom of the grooves) can also be measured and many tabulations of firearm rifling characteristics include it along with the bore diameter.
Number of lands and grooves The number of lands and grooves in the barrel of a rifled firearm can range from as few as two to more than twenty.
Direction of twist of the rifling The rifling in a firearm barrel can spiral in one of two directions. For right twist (or Smith and Wesson) type rifling the thumb of the right hand points in the direction the bullet travels, while the other fingers curl in the direction of the rotation of the bullet. For left twist (or Colt) type rifling the thumb of the left hand points in the direction the bullet travels, while the other fingers curl in the direction of rotation of the bullet.
Degree of twist of the rifling This is the angle made by the lands of the rifling with the axis of the barrel. It is often expressed as the number of inches of barrel required for one complete turn of the rifling.
Class characteristics of cartridges
Shape of firing chamber Cartridges may have straight sides or may be bottle-shaped to fit the firing chamber. Some firing chambers have flutes which are used to ‘float’ the expended cartridge out of the firing chamber; flutes leave characteristic markings on the sides of fired cartridges.
Location of firing pin Modern firearm cartridges are either rimfire or centerfire.
Size and shape of firing pin For 0.22 caliber single-shot ‘boys rifles’ the size and shape of the firing pin is one of the most diagnostic class characteristics. Some other weapons (Beretta 1934, Glock pistols and the Bren gun) produce very distinctive firing pin impressions. For other types of weapon this class characteristic is less valuable.
Size and shape of extractors and ejectors Among automatic and semiautomatic firearms there is considerable variation in the size and shape of the extractor and ejector. For example, the 0.45 caliber M1911A1 semiautomatic pistol has a narrow strip of spring steel for an extractor, whereas the M14 semiautomatic rifle has two broad extractors on either side of the bolt.
Geometrical relationship of extractor and ejector This determines the direction in which expended cartridges are ejected from the weapon. Extractors and ejectors may be 180°, 120° or 90° apart.
Shotguns
The barrels of shotguns lack rifling; they are usually produced from drilled out bar stock or from extruded seamless metal tubing. The diameter of a shotgun barrel is determined by its gauge. The gauge of a barrel is the number of lead balls whose diameters are the same as the interior diameter of the barrel whose total weight is one pound. A 10 gauge shotgun has a bore diameter of 0.775 inch; a 12 gauge shotgun barrel has a diameter of 0.730 inches. In the case of 410 gauge the gauge is the actual bore diameter. Because shotgun barrels lack rifling shotguns will not normally produce markings on the projectiles that they fire. Criminals may saw off the barrel of a shotgun to facilitate concealment. Burrs may be left which will mark the sides of shot columns or the edges of wads. An important feature of shotgun barrels is the presence of a choke or constriction at the muzzle. The purpose of the choke is to concentrate the shot pattern so that enough shotgun pellets strike the game animal to inflict a fatal or incapacitating wound. The degree of constriction is indicated by the designation of the barrel as cylinder bored (no choke), improved cylinder (slight constriction), modified choke (some constriction) and full choke (maximum constriction). The degree of choke of a shotgun barrel is determined by the fraction of pellets falling within a 30-inch (76.2 cm) circle at a range of 40 yards (36.6 m). A cylinder-bore shotgun will place 25-35% of the pellets inside the circle; an improved cylinder barrel 3545%; a modified choke barrel 45-65%; and a full-choke barrel 65-75%. The barrels of double-barreled shotguns usually have different chokes. The choke of a shotgun barrel may be increased by the use of barrel inserts or adjustable compensators (which also direct the propellant gases issuing from the muzzle to the side and rear to reduce recoil). The choke of a shotgun affects the size of the pellet pattern it fires; consequently the choke of a shotgun will have an effect on the determination of the range at which a questioned pellet pattern was fired.
Ammunition
Types of bullets
Bullets are made in a variety of shapes (e.g. round nosed, pointed nose and wad cutter) and of a variety of materials. Lead bullets are made of relatively pure lead; because of their softness they are used in low-velocity firearms (0.22 caliber pistols and rifles). Lead alloy bullets contain a few percent of an alloying element such as antimony (commercially manufactured bullets) or tin (home-cast bullets). Lead alloy bullets are harder than lead bullets and are used in higher muzzle velocity firearms. Both lead and lead alloy bullets readily deform on impact; they may also fragment. Severe deformation and fragmentation can severely handicap the firearms examiner. Lead and lead alloy bullets are also subject to oxidation that can obscure or obliterate surface striations. On the positive side, lead and lead alloy bullets expand to completely fill the rifling of the weapon so that the firearms examiner may have a very large number of striations which can be matched.
Full-metal jacket bullets consist of a lead core covered by a thin sheet of gilding metal, brass, steel or aluminum. Usually the lead core is exposed at the base of the bullet. Full-metal jacket bullets resist deformation and fragmentation better than lead or lead alloy bullets. On the other hand, however, full-metal jacket bullets do not expand to completely fill the weapon’s rifling with the result that such bullets are not as well-marked as lead or lead alloy bullets. Luballoy® bullets consist of a lead core covered by a very thin layer of copper. The copper layer tends to flake off the bullets’ surfaces, taking with it the fine striation patterns.
Semijacketed or partially jacketed bullets also have lead cores; commonly, their sides and bases are covered with a metal jacket, leaving the lead core exposed at the nose. There is a variety of semijacketed bullets, including the following.
1. Semijacketed hollow-point bullets in which the nose of the bullet is hollowed out to facilitate expansion of the bullet. Exploding bullets are hollow points with a percussion cap inserted into the cavity in the nose. Metal spheres may also be inserted in the cavity in the nose to facilitate expansion.
2. Soft point bullets have a soft metal plug placed in the nose of the bullet to facilitate expansion on impact.
3. Metal point bullets have a hardened metal cap on their noses; their sides are not jacketed.
4. Bronze point bullets, in order to facilitate bullet penetration and expansion in thick-skinned game animals, have a pointed bronze casting inserted in the nose of the bullet. The sides of the bullet are normally jacketed.
In addition to the bullet types discussed above there are a number of specialty bullets. Frangible bullets are composed of particles of powdered iron pressed together or particles of powdered iron and lead pressed together with an organic binder. These bullets break up on impact so that they present no danger of ricochet. Frangible bullets are used in shooting galleries and for cattle stunning in slaughterhouses. Gla-ser safety slugs were developed to deal with the hazards associated with ricochet and ‘overpenetration’ of conventional bullets. Conventional bullets which miss their intended human target may ricochet and strike an unintended target; conventional bullets may go through the intended target and strike a second target. The projectile in Glaser safety slugs is a plastic bag filled with shot. When it impacts a target the bag spreads out, distributing the impact forces. At close range the Glaser safety slugs can incapacitate their human target; however, they are unlikely to pass through the body and injure a second person. Armor-piercing bullets have steel jackets which are poorly marked by the rifling of the gun barrel; these bullets may have tungsten carbide cores. Teflon-coated bullets are covered with a layer of self-lubricating fluorocarbon polymer. This layer reduces friction between the bullet surface and the interior of the gun barrel so that these bullets are able to reach very high muzzle velocities. Teflon-coated bullets were designed to aid police officers in situations where criminals have taken shelter behind an automobile; these bullets can penetrate an automotive engine block and can also penetrate soft body armor. Open tubular bullets can also achieve very high muzzle velocities. These bullets consist of an open tube of copper or brass with a plastic cup which acts as a gas check. Open tubular bullets made of brass can penetrate soft body armor.
Discarding sabot rounds have a semi jacketed soft-point bullet pressed into a plastic cup or sabot. The Remington Accelerator cartridge is a 0.30/06 rifle cartridge with a 5.56 mm (0.223 caliber) semi jacketed soft-point bullet. When this type of round is fired the sabot separates from the bullet and eventually falls to the ground. Only the sabot engages the rifling in the weapon’s barrel; therefore, it alone bears class and individual characteristics.
Cartridge cases
Cartridge cases are made of brass, steel or aluminum. They may be plated with nickel to reduce corrosion. Cartridges are made in a variety of shapes (e.g. straight-sided or bottle-shaped) to conform to the different shapes of the firing chambers of firearms. There are a number of broad categories of cartridge shapes.
1. Belted cartridges are high-powered rifle cartridges. They have a raised belt at the base of the cartridge to control the positioning of the cartridge in the firing chamber. Belted cartridges may be rimmed or rimless (see below).
2. Rimmed cartridges have a rim at the base. Rimmed cartridges are used in revolvers; the rims prevent the cartridges from sliding forward out of the firing chambers of the revolver’s cylinder.
3. Rimless cartridges have a base that is the same diameter as that of the body of the cartridge. Rimless cartridges have an extractor groove circling the base of the cartridge. The extractor of the firing weapon engages this groove so that the expended cartridge can be extracted from the weapon.
4. Semirimmed cartridges have an extractor groove like that found in rimless cartridges; the base of the cartridge has a larger diameter than the body of the cartridge.
5. Rebated (or rebated-rimless) cartridges resemble rimless cartridges in that they have an extractor groove; however, the base of the cartridge has a diameter smaller than the body of the cartridge.
Cartridges can also be categorized by the location of the primer that is used to initiate the propellant in the cartridge. In rimfire cartridges the primer compound is placed inside the rolled rim of the cartridge; the weapon’s firing pin strikes the rim of the cartridge to fire it. Centerfire cartridges have a depression in the center of the cartridge for a percussion cap made of gilding metal. Boxer cartridges are used in the United States; these have a cylindrical depression in the center of the base of the cartridge for the percussion cap with a flashhole leading to the powder. The Boxer primer contains a small metal anvil against which the firing pin crushes the primer compound. Berdan cartridges are used in Europe and other parts of the world. The Berdan cartridge has a cylindrical depression in its base. There is a small cone in the center of the depression through which three flashholes lead to the powder. The cone fulfills the role of the anvil in the Boxer primer; the firing pin crushes the primer compound against the cone. The base of the cartridge bears a headstamp. The headstamp indicates the vendor of the cartridge and the type of cartridge. The headstamp on a rifle cartridge might read ‘REM-UMC 30-06 SPRG’ which would reveal that the cartridge was manufactured by Remington-United Metallic Cartridge Company and that it is a bottle-shaped cartridge introduced in 1906 for 0.30 caliber Springfield rifles.
Shotshells
The bodies of shotshells are made of coated paper or plastic. They have bases made of brass. Shotshells have headstamps like the cartridges used in other types of firearms. Shotgun ammunition normally contains a mass of small pellets. The pellets may be lead, lead alloy (‘chilled’ shot) or stainless steel. Their diameters vary from 0.36 inches (000 buckshot) down to 0.05 inches (No. 12 birdshot). The number of pellets per shotshell is related to their size and the gauge of the shotgun. For example, 12 gauge 00 buckshot shotshells contain nine or twelve 00 buckshot, whereas 12 gauge no. 2 shotshells may contain over a hundred no. 2 birdshot. Wads are disks of cardboard, thin wood or plastic that are used to hold the shot in place inside the shotshell. One wad will typically separate the shot from the propellant (the overpowder wad); another wad may be placed on top of the shot mass at the mouth of the shotshell (the overshot wad). In many shotshells the wads may be replaced by plastic one- or two-piece shot columns. These shot columns have plastic cups that enclose the shot and protect them from deformation due to contact with the interior of the shotgun barrel. Plastic particles may be packed with the shot to provide similar cushioning. Wads and shot columns may be found inside shotgun wounds inflicted at close range. Patterned contusions caused by wads and shot columns may occasionally be seen adjacent to shotgun wounds inflicted at ranges of several feet.
Shotshells may also contain single projectiles such as round balls or rifled slugs. The single round balls fired by shotguns are essentially the same as the round balls once fired in smoothbore muskets. Rifled slugs are hollow-base conical lead slugs with projecting fins. The fins provide stabilization for the slugs so that they are more accurate than round balls. The wounding effects of rifled shotgun slugs are similar to those produced by the Minie bullets (or minnie balls) used during the American Civil War.
Powders
The original propellant used in firearms was black powder (the name being a translation of Schwartz-pulver, a propellant formulation named for Berthold Schwartz), originally a mixture of charcoal, sulfur and potassium nitrate. Black powder was used as the propellent in firearms until the end of the nineteenth century when it was gradually replaced by smokeless powder. Hobbyists (black powder enthusiasts, Civil War re-enactors and the like) still use black powder or black powder substitutes (e.g. sulfurless black powder or Pyrodex) as propellants. Smokeless powders contain nitrocellulose. Single-base smokeless powders contain nitrocellulose combined with various additives (e.g. potassium sulfate, diphenylamine and dini-trotoluene) which affect the mechanical or chemical properties of the powder grains. Double-base smokeless powders contain nitroglycerin as an additional energetic material. The grains of smokeless powders are produced either by the extrusion process or by the ball powder process. In the extrusion process, the ingredients of the propellant are mixed together in a cake with a volatile solvent. The cake is forced against a steel plate in which a number of holes have been made. The propellant mixture is extruded through these holes and a rotating metal blade cuts off short lengths of extruded material. Extruded propellant grains may take a variety of forms: rods, perforated rods, flattened disks, perforated disks or flakes. The ball powder process also involves an extrusion process, except that the propellant grains are extruded into hot water where they assume spherical shapes. The spheres may be further processed by flattening them between metal rollers. Propellants produced by the ball powder process are either spherical or flattened disks.
Propellants for firearms are manufactured in batches. The manufacturer tests each batch to determine its ballistic performance. To achieve the ballistic performance desired for the final product two or more batches of powder will be blended together. Thus, the powder in a cartridge or shotshell will consist of grains from different batches having slightly different chemical compositions.
