Cartridge Anatomy: Case, Primer, Propellant, Projectile

The cartridge case serves three simultaneous functions: it holds all the other components in correct geometric relationship until the moment of firing; it seals the chamber against the rearward escape of propellant gas during combustion (obturation); and it provides the mechanical anchor point by which the extractor withdraws the spent case from the chamber afterward. Getting this right requires a tight tolerance on case geometry relative to the chamber it will be fired in.

Case materials. Brass (typically cartridge brass, 70 percent copper and 30 percent zinc) is the dominant material because it work-hardens during drawing, resists the corrosive combustion products of primer and propellant, and springs back just enough after firing to release from the chamber wall without sticking. Steel cases (lacquer-coated, polymer-coated, or zinc-plated) are standard in military surplus and lower-cost commercial production. Soviet and Russian military cartridges (7.62x39mm, 5.45x39mm, 7.62x54mmR) are nearly always steel-cased; Chinese military production similarly uses steel. Aluminium cases appear in some specialty commercial lines (CCI Blazer aluminium) and are not reloadable, which affects evidence-assessment reasoning when reload evidence is relevant to case timelines. Steel and aluminium cases do not spring back as readily as brass and tend to remain in contact with the chamber wall after firing, producing more pronounced extractor and ejector marks on the rim.

Rim classification. Five rim geometries define the mechanical interface with the extractor:

• Rimmed: the case head has a pronounced flange (the rim) that extends beyond the body diameter. Examples include .38 Special, .357 Magnum, .44 Magnum, and the .303 British that was the Indian Army's standard cartridge from the 1880s through to Independence. Rimmed cartridges extract reliably from revolvers and break-action rifles because the rim is the headspace reference; they stack poorly in detachable box magazines and are relatively rare in modern semi-automatic pistol or rifle designs.
• Semi-rimmed: the rim extends slightly beyond the body and an extractor groove is cut ahead of it. The .38 ACP, .32 ACP, and .38 Super are semi-rimmed. Semi-rimmed cases appear infrequently in casework.
• Rimless: the rim diameter equals the body diameter, and the extractor groove is recessed into the head. The 9x19mm Parabellum, .45 ACP, .308 Winchester (7.62x51mm NATO), and 5.56x45mm NATO are rimless. This is the dominant class in modern centre-fire semi-automatic pistol and rifle ammunition. Headspace is controlled by the case mouth resting on the chamber mouth.
• Semi-rimless: a transitional geometry where the rim is slightly larger than the body but a definite extractor groove is present. The .32 Winchester Self-Loading (rare in casework) is semi-rimless; the class is essentially historical.
• Belted: a raised belt ahead of the extractor groove provides the headspace reference and controls how far the case seats in the chamber. The belt is a legacy of early large-calibre cartridges where the case body taper was insufficient to control headspace on its own. .300 Winchester Magnum, .338 Lapua Magnum, and .50 BMG (12.7x99mm NATO) are belted. In Indian contexts, .300 Win Mag appears in sniper and long-range target-rifle casework.

Wall thickness and taper. The case walls taper from mouth to head; a standard 9x19mm Parabellum case has a wall thickness of approximately 0.43 mm at the case mouth and 0.53 mm at the head. Taper allows the case to release from the chamber after firing (aided by the elastic springback of brass). When cases are found double-loaded or damaged by overpressure, the wall geometry is distorted in characteristic ways that the examiner documents: bulged case bodies indicate unsupported-chamber firing (loose headspace), split case mouths indicate overpressure or deep-seated bullets, and swaged-case heads indicate chamber or barrel catastrophic failure.

The primer pocket is machined into the case head to accept the primer cup. All modern centrefire cartridges use either a Berdan or a Boxer primer pocket architecture, and the distinction has operational significance in forensic examination because it affects whether a case can be reloaded and therefore whether the presence of a reloaded cartridge at a scene is probatively meaningful.

Boxer primer pocket (named after British Colonel Edward Mounier Boxer, who patented the design in 1866) has a single, centrally placed flash hole in the base of the primer pocket. The anvil that the primer compound is squeezed against when the firing pin strikes the cup is integral to the primer itself, not a feature of the case. This architecture allows the spent primer to be punched out through the flash hole with a standard decapping pin, making the case straightforward to hand-reload. Nearly all US commercial cartridges (Winchester, Remington, Federal, Speer, Hornady) use Boxer primer pockets. Federal Cartridge's Gold Medal Match 308 Winchester, Winchester Ranger T-Series 9mm, and Hornady Critical Duty 9mm +P all use Boxer architecture.

Berdan primer pocket (named after American Hiram Berdan, who patented this design in 1866 in parallel with Boxer) has the anvil machined as an integral post in the base of the primer pocket, and the flash holes are two small offset ports flanking the anvil. The Berdan primer cup itself is simpler (no integrated anvil) and performs reliably under the higher chamber pressures typical of military rifle cartridges. Most European commercial ammunition, most Russian and Eastern European military cartridges, and most Indian Ordnance Factory production uses Berdan priming. S+B (Sellier and Bellot, Czech Republic) commercial production is Berdan-primed. IOF 7.62x51mm NATO production for Indian Army service and IOF 9x19mm for CRPF use Berdan pockets. The anvil post is visible as a raised feature inside the primer pocket when the primer is removed; the two small flash holes are visible on either side of the anvil.

For the forensic examiner, the Berdan/Boxer distinction is relevant in at least three evidence contexts: (1) whether a recovered fired case could have been home-reloaded with consumer equipment (Berdan decapping requires a specialised tool or hydraulic decapping and is uncommon among amateur reloaders); (2) whether the cartridge's probable production origin is US/Western European commercial or Eastern European/military/IOF; and (3) whether a suspected reload with the wrong primer size (large rifle primer seated in a small rifle pocket or vice versa) contributes to a misfeed or misfire that is part of a shooting-incident reconstruction.

Primer cup dimensions. Centrefire pistol and rifle primers come in four nominal sizes: small pistol (0.175 inch diameter), large pistol (0.210 inch), small rifle (0.175 inch, slightly different seating depth from small pistol), and large rifle (0.210 inch). Military primers are often identical dimensions but are sealed at the primer/pocket junction with a sealant (visible as a coloured lacquer ring) to prevent moisture ingress and cook-off migration under tropical and field conditions. IOF primer sealant is typically red or maroon; NATO production varies between red, green, and purple sealants per manufacturer.

Modern propellant is smokeless powder, meaning it is based on nitrocellulose (single-base), nitrocellulose plus nitroglycerin (double-base), or nitrocellulose plus nitroglycerin plus nitroguanidine (triple-base) chemistry. Black powder is confined to muzzle-loading weapons and historical propellant comparisons in casework involving antique weapons. This section covers the physical form and charge weight of the propellant that an examiner may encounter in an unburned or partially burned state inside a recovered cartridge or as residue after firing.

Grain shapes and surface area. Propellant is manufactured in discrete granules called grains. The shape of those grains determines the surface-area-to-volume ratio and therefore the burn-rate profile as the grain combusts. Three shapes dominate:

• Ball (spherical or slightly flattened ball): used in pistol propellants (Alliant Power Pistol, Hodgdon H110) and some rifle propellants (Hodgdon H335, Winchester 748). Ball powder is compact, meters consistently through powder dispensers, and has a relatively high loading density. When recovered partially burned, ball grains are recognisable as small spheres or flattened discs, typically 0.3 to 1.0 mm in diameter.
• Flake (flat disc or platelets): thin flat grains used in fast-burning pistol and shotgun propellants (Alliant Bullseye, Green Dot, Red Dot). Fast burn rate makes flake powders suitable for pistol calibres (.38 Special, 9x19mm, .45 ACP) where the short barrel gives the gas pressure only a brief time to act on the projectile. Partially burned flake grains appear as small, flat irregular platelets.
• Extruded / stick (cylindrical rods): used in medium- and slow-burning rifle propellants (Hodgdon Varget, IMR 4350, IMR 4831, Vihtavuori N560). Single-perforation or multi-perforation cylinders provide progressive burning as the outer surface area increases when the outer layer burns away and the holes enlarge inward. Stick powder is found in nearly all high-pressure rifle calibres including 5.56x45mm NATO, 7.62x51mm NATO, and .308 Winchester.

Charge weights. A standard 9x19mm Parabellum 124 gr FMJ load uses approximately 5.0 to 6.5 grains of propellant depending on the specific powder and pressure target (SAAMI maximum is 35,000 psi). A .45 ACP 230 gr FMJ load uses approximately 5.0 to 6.0 grains of a faster-burning propellant (SAAMI maximum 21,000 psi). A 5.56x45mm NATO M193 55 gr load uses approximately 25 to 27 grains of extruded powder (NATO maximum 55,000 psi, SAAMI .223 Rem maximum 55,000 psi). A 7.62x51mm NATO 147 gr load uses approximately 42 to 45 grains of extruded powder (NATO maximum 60,191 psi; SAAMI .308 Winchester maximum 62,000 psi). The .50 BMG (12.7x99mm NATO) uses approximately 230 to 240 grains of a large-cylinder extruded powder (SAAMI maximum 54,000 psi).

Forensic relevance. Unburned or partially burned propellant grains recovered from wounds, garments, or scene surfaces can be examined microscopically to establish: (a) grain shape and therefore the class of propellant used; (b) whether coating materials (deterrent coatings such as dibutyl phthalate that moderate burn rate) are present and match a known production lot; and (c) the approximate charge weight from grains recovered and grain density. This supports range-of-fire estimation (intermediate-range firing produces significantly more unburned grain deposition than contact or distant firing) and, where intact cartridges are available for comparison, establishes whether the fired round came from the same production lot as a suspect's possession.

The projectile is the component that the examiner most frequently recovers as physical evidence, because it is what impacts a surface, a body, or a backstop and remains there while the case is ejected. The projectile's physical characteristics record the rifling marks of the weapon that fired it, the deformation from the target it impacted, and the manufacturing decisions of whoever made it.

Full metal jacket (FMJ). The standard military projectile under the 1899 Hague Declaration (which prohibits the use of expanding projectiles against combatants in armed conflict) is the full metal jacket, in which a gilding-metal jacket (90 percent copper, 10 percent zinc) encloses a lead core completely, including a closed base. The jacket prevents lead fouling in the barrel, maintains shape during feeding and chambering, and allows the bullet to travel through tissue without expansion. FMJ projectiles are the standard for military service ammunition worldwide: M193 5.56 55 gr, M855 5.56 62 gr, and 7.62x51mm NATO 147 gr are all FMJ or close variants. IOF 9x19mm for CRPF service is FMJ.

Jacket materials and construction. Most commercial FMJ uses gilding metal (90Cu/10Zn). Some military and surplus production uses a steel core jacket (magnetic, identifiable with a magnet) or a bimetallic jacket (steel-core bullet in a gilding-metal-clad jacket). Russian and Eastern European 7.62x39mm AK-pattern ammunition is typically bimetallic jacketed with a steel core, and its magnetic attraction to a magnet is a quick identification indicator on scene. Indian M855-equivalent production and AK-47-compatible ammunition seized in Naxalite operations or Indian-Myanmar border interdictions frequently shows steel-core, bimetallic jacket construction.

Projectile weight in grains. The most common metric for projectile weight in the US and UK commercial context is grains (gr), where 1 grain equals approximately 0.0648 grams. A 9x19mm Parabellum bullet is typically 115, 124, or 147 gr (7.45, 8.04, or 9.52 g). A .45 ACP bullet is typically 185 or 230 gr (12.0 or 14.9 g). A 5.56x45mm NATO bullet is 55 or 62 gr (3.56 or 4.02 g). A 7.62x51mm NATO bullet is 147 gr (9.53 g). A .50 BMG bullet is 647 or 660 gr (42.0 or 42.8 g). The examiner measures the weight of a recovered fired bullet on a laboratory balance and uses this as one of the first classification variables when the calibre is not yet established.

Jacket-core bonding. In standard FMJ and conventional soft-point bullets, the jacket is pressed around the lead core but not chemically bonded to it. At impact velocities above approximately 1,400 fps in tissue (common for rifle rounds, and for high-velocity pistol rounds at close range), the jacket and core can separate, scattering jacket fragments and the deformed core in different anatomical planes. This jacket-core separation is a complicating factor in wound-track reconstruction and in recovering intact evidence for striation comparison. Bonded-core designs (Speer Gold Dot uses electrochemical bonding, Federal HST uses bonding between the plug and the rear section) are covered in the specialised ammunition topic.

Each of the four components is manufactured to dimensional tolerances, and the cartridge as a whole is assembled to a specification that must fit within the chamber dimensions of the intended weapon. Deviations in either the cartridge or the chamber produce characteristic failure modes that the examiner encounters as dud rounds, damaged cases, and components recovered from wounds or scenes.

SAAMI and CIP tolerance standards. SAAMI and CIP each publish chamber and cartridge drawings for the cartridges they standardise. SAAMI governs US commercial and military production; CIP governs European commercial production and many military calibres under Permanent International Commission proof authority. For the same nominal calibre, SAAMI and CIP specifications may differ slightly. The 9x19mm Parabellum SAAMI maximum cartridge overall length (COL) is 1.169 inches; the CIP COAL maximum is 29.69 mm (1.169 inches). The maximum average pressure (MAP) for 9x19mm Parabellum under SAAMI is 35,000 psi; under CIP the corresponding figure for the same nominal cartridge is 34,809 psi (240 MPa), essentially identical. For .308 Winchester vs 7.62x51mm NATO, the nominal pressures differ meaningfully: SAAMI .308 Win MAP is 62,000 psi, while NATO STANAG 2310 specifies the interchangeability requirements for 7.62x51mm NATO ammunition, with a maximum pressure of approximately 60,191 psi (415 MPa) under EPVAT measurement. Firing a .308 Winchester commercial-spec load in a headspace-marginal 7.62 NATO chamber can produce a stretched case or, in extreme cases, a case-head rupture.

Squib loads. A squib is a cartridge that fires the primer but ignites only a partial propellant burn or none at all, producing insufficient pressure to fully exit the projectile from the barrel. The projectile lodges in the barrel, typically within 1 to 6 inches of the chamber. The case shows a normal primer strike and usually a partially flattened primer but minimal case expansion. If a subsequent round is fired before the squib is detected, the barrel obstruction produces a catastrophic barrel failure whose debris pattern and the deformed squib projectile recovered from the barrel are both examined and documented by the laboratory. Squib events in casework arise from defective commercial rounds, improperly assembled reloads, or deliberately sabotaged ammunition.

Hangfire. A hangfire is an abnormally delayed ignition: the primer strikes normally but full powder ignition is delayed by milliseconds to seconds. Hangfire is distinguished from a misfire (no ignition at all) by the fact that a round submitted to the laboratory after a suspected hangfire may still contain an unfired or only partially fired powder charge and an unstruck or struck primer. In India, hangfire documentation follows Indian Army Ordnance Corps procedures; the UK MOD Quality Assurance Directorate maintains a failure-mode reporting system for all Service cartridges.

Slamfire. A slamfire is the opposite failure: the weapon fires before the trigger is deliberately pulled, typically when the bolt or slide chambers a round and the inertia of the firing pin, or a defective disconnector, causes ignition. The case from a slamfire shows a firing-pin indent, but often one that is offset or double-struck if the weapon continued cycling in full-automatic mode. Slamfire cases may show marks from multiple chambering cycles because the weapon continued to cycle and the case was partially ejected and re-chambered.

In the Mumbai 26/11 (November 2008) attacks investigation, forensic examination of hundreds of 7.62x39mm fired cases recovered from multiple sites established that the AK-pattern ammunition used by the attackers was a consistent lot from a single production source, attributable by headstamp and case metallurgy. The case anatomy examination contributed to source attribution before full microscopic comparison of striation marks. This is typical of the multi-stage examination sequence: case anatomy examination is the first phase that narrows the field before the more time-intensive microscopic comparison begins.

What the fired case records. The base of the fired case records: the firing-pin impression shape and depth; the geometry of the breech face as impressed into the case head during firing; the primer-pocket expansion as a function of chamber pressure; the presence or absence of Berdan or Boxer flash-hole architecture; and the extractor groove wear and ejector-mark deformation. The case mouth and body record: the chamber diameter (through case expansion to the chamber wall during firing); any feeding ramp marks from the magazine or feed ramp; and the neck-sizing pattern if the case was reloaded.

Headspace and case stretching. When headspace (the distance from the bolt face to the datum line in the chamber at which the case rests for firing) is excessive, the case stretches longitudinally during firing because the brass is pulled forward by propellant pressure and the head is held at the bolt face. A stretched case shows a bright annular ring of thinned brass approximately 0.25 to 0.5 inches forward of the case head. Visible case stretching indicates the firearm had marginal or excessive headspace, which is a mechanical condition of the weapon that the forensic examiner documents and that may be relevant to whether the weapon was safe for use or likely to have produced accidental discharge.

India-specific context. In Tihar Jail weapon-smuggling cases and Naxalite-interdiction cases in Chhattisgarh, Jharkhand, and Odisha, the examination of 7.62x39mm cases from AK-pattern weapons involves identifying whether the cases are Soviet-era military, Chinese military, Romanian commercial, or Indian katta-derived custom load (locally primed). The combination of case geometry, Berdan vs Boxer primer pocket, headstamp origin, and primer sealant colour provides the initial matrix for that differentiation before laboratory instrumentation is applied. UK forensic firearms casework (NABIS, the National Ballistics Intelligence Service) follows similar case-documentation protocols, and the Crown Office in Scotland requires a formal dimensional comparison report before a case is attributed to a specific weapon type.