Cartridge-Case Markers: Firing Pin, Breech Face, Ejector and Extractor

When the firing pin strikes the primer cup, the pin's tip is driven into the cup face with enough force to compress the priming compound against the anvil and initiate deflagration. In doing so, the pin's tip leaves an impression in the soft metal of the primer cup. The shape of that impression is a class characteristic determined by the tip geometry of the firing pin: circular in the vast majority of semi-automatic pistols and revolvers, oval in some designs where the pin has a transverse axis asymmetry, rectangular in some bolt-action rifles that use flat-faced firing pins, and occasionally showing a drag mark where the pin strikes offset and drags across the cup face before the action opens.

The size and depth of the firing-pin impression are also class characteristics. Firearms that generate high peak chamber pressures tend to produce deeper impressions because the primer cup is pushed rearward onto the pin with greater force at the instant of firing. Conversely, a light firing-pin spring in a worn or deliberately lightened trigger action may leave a shallow impression, a potential indicator of a modified action.

Individual characteristics of the firing-pin impression arise from the microscale tooling marks on the tip of the pin itself, from any chipping, pitting, or corrosion that has developed at the tip during service, and from the asymmetry of any wear pattern that causes the pin to strike consistently off-centre. These individual markings appear on the primer cup surface as striations, pits, or raised features surrounding the central impression. They are the basis for any identification conclusion linking a fired case to a specific weapon.

In the United States, the FBI Reference Firearms Database includes firing-pin impression shape as a categorisation field, allowing GRC-level lookups on case exhibits before any individual comparison is attempted. The FBI Laboratory's Firearm and Toolmark Unit SOPs require that firing-pin impression shape and diameter be recorded in the case notes for every cartridge case submitted.

In Germany, BKA examiners operating under the ENFSI Firearms Working Group BFW01 guideline record firing-pin impression shape using a standardised category list: circular, oval, rectangular, pointed-oval, and miscellaneous. This categorical field feeds directly into the IBIS database indexing at BKA's EBIS terminal, so that automated search results are pre-filtered by firing-pin class before the algorithm ranks candidates by topographic similarity.

In India, CFSL Chandigarh examiners follow an SOP that records firing-pin impression shape and approximate diameter in millimetres (typically 1.0 to 2.5 mm for pistol primers, 2.0 to 3.5 mm for rifle primers) as the opening GRC step in cartridge case examination. The 9 mm Parabellum cases most commonly received from police submissions show circular firing-pin impressions with diameters characteristic of the firing weapon's pin design; Glock firing-pin tip geometry (a smaller-diameter circular impression with a smooth periphery) is distinguishable from 1911-type round-nose firing-pin impressions at the comparison microscope.

The breech face is the forward surface of the bolt, slide, or breech block that seals the chamber opening when the cartridge is chambered. During firing, the propellant gas drives the cartridge rearward into the breech face with enormous force (peak chamber pressures in 9 mm Parabellum reach maximum average values of approximately 235 MPa (CIP standard) to 241 MPa (SAAMI standard)). The soft brass case head deforms slightly and flows into the surface texture of the breech face, taking an impression of the machining marks, grinding lines, tool-path striations, and any service-acquired features (scratches, pits, hardened surface treatments that have worn unevenly) present on that face.

The breech-face impression is distributed across the primer area of the case and into the surrounding case head material. In semi-automatic pistols with a striker-fired mechanism (Glock, SIG Sauer P320, HK VP9), the breech face is the flat surface of the slide; in hammer-fired pistols (Beretta 92, CZ 75, 1911 platform), it is the bolt face or barrel extension; in bolt-action rifles, it is the recessed bolt face. Each configuration produces a slightly different geometry of impression, which serves as a class characteristic.

The individual characteristics of the breech face arise from the unique tool-path pattern of the milling or grinding operation that produced it. No two breech faces, even machined to identical specifications on the same tooling, share an identical surface texture at the microscale. The striations and raised features on the breech face are transferred to the case as a matching pattern of pits and raised features. This transferred pattern is compared in the comparison microscope split field, looking for corresponding arrays of features across the evidence case and the test-fire case.

In UK NABIS casework, breech-face comparison is the primary individualisation tool for cartridge cases where the firing-pin impression has been damaged or where a pistol type is identified by ejector geometry. The BalliScan 3D platform at NABIS-accredited laboratories captures the full case head topography, allowing the digital comparison of breech-face impressions in the IBIS TRAX-3D software environment.

In the European EBIS network, the BKA protocol requires that breech-face impressions be described by the machining pattern (concentric, radial, random, or a specific tool-path orientation if identifiable). This categorical entry, combined with firing-pin shape, forms the primary filter for EBIS automated search pre-screening.

The ejector is the mechanism that knocks the fired case clear of the action after the bolt or slide has moved rearward on extraction. In most semi-automatic pistols, the ejector is a fixed or pivoted pin located inside the frame that strikes the base of the case as the slide retracts, flipping the case out of the ejection port. The ejector contacts the case rim (on rimmed or semi-rimmed cases) or the case head groove area at a specific geometric location: typically at approximately the 9 o'clock position on the case base when the firearm is viewed from the rear in right-hand-ejection designs. This convention is described in the AFTE Glossary 6th edition (2013) as a standard clock-position reference for documentation.

The ejector mark is a striated or impressed feature at this contact location. Because the ejector tip is machined to a specific geometry and accumulates individual surface features through service wear, the ejector mark on a fired case carries both class characteristics (the ejector tip shape and location relative to the case base) and individual characteristics (the microscale topography of the ejector tip face that is transferred to the case).

The extractor is the spring-loaded claw that grips the cartridge case head during extraction, pulling the fired case out of the chamber as the bolt or slide moves rearward. The extractor contacts the extractor groove machined into the case head at a position typically at the 3 o'clock location (opposite the ejector). Extractor marks appear on the extractor groove face and on the rim or groove lip where the claw bites during the extraction stroke. They are striated in nature, produced by the relative motion of the extractor tip across the case metal as the case is pulled from the chamber.

Magazine-lip marks are an additional category seen on the base of cartridges that were loaded in a detachable box magazine. As the cartridge is stripped from the magazine by the slide or bolt, the magazine lips contact the case below the extractor groove, leaving a pair of parallel striated marks. These marks are class-characteristic of the magazine type and can help confirm whether a specific magazine was used, if the magazine is recovered.

The AFTE-recommended comparison protocol for cartridge cases specifies a defined order of examination that prevents the confirmation bias that would arise if an examiner focused immediately on the most visually striking feature without first confirming class consistency.

Step 1 is calibre and headstamp verification. The examiner confirms that the evidence case and the test-fire case from the suspect weapon are the same calibre and, where possible, share the same headstamp manufacturer. A mismatch in calibre is an immediate elimination. A headstamp mismatch does not eliminate (the same weapon fires cartridges from different manufacturers) but is noted for the case file.

Step 2 is firing-pin impression class comparison: shape and approximate diameter. A shape mismatch (e.g. circular evidence impression vs rectangular test-fire impression) is an elimination. Matching class firing-pin shape advances the comparison to the next step.

Step 3 is breech-face pattern class comparison: the overall machining pattern (concentric, radial, or random) and any large-scale tool-path direction are compared. Gross mismatch eliminates.

Step 4 is ejector mark position comparison. The clock position of the ejector mark on the evidence case is compared against the test-fire case. A position difference of more than a few degrees is a class-level mismatch that may indicate a different firearm design.

Step 5 is individual-characteristic comparison, where the examiner moves to the comparison microscope and aligns the firing-pin impressions in the split field, looking for corresponding surface features. This is followed by alignment of breech-face areas and ejector mark areas.

Step 6 is documentation: photomicrographs of each alignment showing the matching or non-matching features in the split field are captured and retained in the case file. The AFTE Glossary 6th edition specifies that documentation must be sufficient for a second examiner to evaluate the comparison independently from the images alone, without needing to re-examine the physical specimens.

1. 1. Calibre and headstamp verification
   Confirm calibre identity. Note headstamp manufacturer and year code. Calibre mismatch is an immediate elimination; headstamp mismatch is noted but does not eliminate.
2. 2. Firing-pin impression class comparison
   Compare shape (circular, oval, rectangular) and approximate diameter. Shape mismatch eliminates. Shape match advances to step 3.
3. 3. Breech-face pattern class comparison
   Compare overall machining pattern (concentric, radial, random). Gross pattern mismatch eliminates.
4. 4. Ejector mark position comparison
   Measure clock position of ejector mark on both cases. Position mismatch beyond tolerance may indicate different firearm design.
5. 5. Individual-characteristic comparison at the comparison microscope
   Align firing-pin impressions in split field. Assess for corresponding surface features (striations, pits, raised marks). Repeat for breech-face area and ejector mark area.
6. 6. Documentation and conclusion
   Capture photomicrographs of each aligned comparison. Apply AFTE Theory of Identification criteria. State conclusion (identification, inconclusive, or elimination) with supporting CMS count or feature-correspondence description.

In the United States, the ATF NIBIN network's automated correlation algorithm (IBIS BrassTRAX-3D HD) processes cartridge case exhibits by capturing topographic maps of the firing-pin impression, breech-face area, and ejector mark area. The algorithm ranks candidate pairings by correlation score. ATF examiners then evaluate the top-ranked candidates on the comparison microscope to confirm or eliminate the machine-generated leads. The NIBIN workflow has processed over 3.7 million acquisitions of ballistic evidence since the system's deployment in 1999, generating over 145,000 confirmed hits linking cases across multiple crimes. The 2020 Operation LEGEND multi-city deployment demonstrated NIBIN's utility in rapidly identifying firearms used across jurisdictions in a short time window.

In the United Kingdom, NABIS receives cartridge cases from police forces across England, Wales, Scotland, and Northern Ireland. The NABIS protocol feeds every exhibit into the BalliScan 3D capture system, which generates a topographic record that is uploaded to the IBIS BrassTRAX comparison engine. NABIS examiner review follows the same AFTE comparison protocol, adapted to the UK Forensic Science Regulator's conclusion framework. Operation Stronghold (2018) demonstrated NABIS linking capability: a series of shooting incidents across Greater Manchester and Merseyside were connected to a single firearm through cartridge case comparisons within 72 hours of the last incident.

In Germany, BKA operates the German EBIS terminal as part of the European Ballistic Information Service network coordinated through ENFSI. When a cartridge case submitted to a BKA examiner yields a correlation score above the BKA threshold in the EBIS database, the examiner conducts an optical comparison on the Projectina PAG II or the Leica FSC comparison microscope. The BKA also accepts cross-border submissions via Europol's firearm-trafficking reporting mechanism.

In India, CFSL Chandigarh has submitted data to INTERPOL's Ballistic Information Network (IBIN) under India's engagement with INTERPOL's Firearms Programme. The CFSL firearms division processes cartridge cases from Punjab, Haryana, and Jammu and Kashmir police submissions, with the most frequent case types being post-encounter cartridge cases from security-force actions along the Pakistan border and urban crime-scene cases involving 9 mm, .32 ACP, and 7.62 Tokarev calibre weapons. The CFSL case reports document all six AFTE comparison steps and include photomicrographic exhibits that become part of the court record under BSA § 39 (opinion of experts, replacing IEA § 45).