Forensic Examination of Firearms, Bullets, Shells and Cartridges

Calvin Goddard, working with Philip Gravelle, popularised the side-by-side comparison microscope in 1925 and used it in the identification of the Sacco and Vanzetti bullets and, later, the St Valentine's Day Massacre casings. The instrument is two monocular microscopes mounted on a shared base, each with its own rotatable stage and objective, joined by an optical bridge that merges the two image halves into a single split field viewed through a binocular eyepiece. The examiner mounts the questioned bullet on one stage, the test-fired bullet on the other, rotates both stages until the lands and grooves align, and looks for striation matches across the seam.

Standard magnification for bullet comparison is 10x to 40x. Higher magnifications wash out the contrast on the brass jacket; lower magnifications miss fine striations. Modern instruments add coaxial fibre-optic illumination so the lighting on both halves is identical, because any difference in shadow direction kills the comparison. The dual-stage rotation is the single feature that distinguishes a comparison microscope from a pair of stereo microscopes side by side. The book chapter on tool mark evidence types and comparisonuses the same instrument for chisel, screwdriver and bolt-cutter marks.

The conclusion at the eyepiece is binary in principle (does the striation pattern reproduce across the seam?) and probabilistic in practice (how many consecutive matching striae, on how many lands, with what known sub-class component?). The AFTE scale below converts that judgement into court vocabulary.

A fired bullet recovered from a body or a wall carries three layers of information that the examiner reads in sequence.

Class characteristics are set when the barrel is manufactured. The four examiners-testable ones are caliber (nominal bore diameter, for example.32 or 9 mm), the number of lands and grooves cut into the rifling, the direction of twist (Right or Left) and the width of the groove impressions on the bullet body. A bullet showing six grooves with a right twist (notation 6R) excludes every weapon with a different rifling specification. Class characteristics narrow the candidate population; they never identify a single weapon.

Indicative land and groove counts for common arms tested in MCQs:

Sub-class characteristics arise when the rifling tool (a broach, a button or an electrochemical cutter) wears progressively through a batch of barrels. A small group of consecutively manufactured barrels can share the same machining irregularity. The examiner must consciously exclude any candidate match that sits on a known sub-class feature before claiming identification, or the defence will reasonably argue that the match is shared across the production batch.

Individual characteristics are random microscopic imperfections in the rifling left by the manufacturing process and subsequently modified by use, fouling and corrosion. They appear as fine longitudinal striations on the bullet body, running parallel to the bullet's long axis on each land impression. The reproducibility of these striations on test-fired bullets from the same weapon is what makes identification possible. AFTE-trained examiners look for consecutive matching striae (CMS) across several land impressions before calling an identification.

Test firing for comparison is done into a water tank (most common in Indian SFSLs because water is cheap and recovers the bullet undeformed), a long cotton-bale trap or a rubber-projectile box. The recovered bullet is mounted on the right stage of the comparison microscope alongside the questioned bullet on the left.

A spent cartridge case carries more identifiable marks than a fired bullet, because the soft brass head is in metal-to-metal contact with five distinct surfaces of the weapon during the firing cycle.

1. Firing-pin impression is the dent in the centre of the primer cup left by the firing pin striking the primer. Its shape (hemispherical, elliptical, rectangular, drag-tail) is a class characteristic of the weapon design. Eccentricity (whether the dent sits dead-centre or off-centre on the primer) and tool-mark detail inside the impression are individual characteristics.
2. Breech-face marks are the parallel or contoured striations transferred from the machining marks on the breech face onto the primer cup and (sometimes) the case head when chamber pressure slams the case rearward.
3. Extractor marks are scrapes on the rim or extractor groove of the case made by the extractor hook as it pulls the case out of the chamber. Location: rim, 9 to 11 o'clock or 1 to 3 o'clock depending on weapon.
4. Ejector marks are impact bruises on the case head where the ejector strikes the rim to throw the case clear of the action. Location: case head, opposite the extractor.
5. Chamber marks are longitudinal scratches on the case body from the chamber walls, particularly on weapons with rough or fouled chambers. Useful when other marks are damaged.

Bolt-action and semi-auto weapons add magazine-lip marks on the case body (from chambering) and feed-ramp marks near the case mouth. These are less individualising but help confirm the action type.

The examiner photographs the case head with oblique low-angle lighting that throws each mark into shadow, then mounts the case on the comparison microscope stage alongside a test-fired case. Firing-pin and breech-face marks deliver most identifications; extractor and ejector marks are corroborative.

Shotguns are smoothbore, so a fired pellet or slug does not carry rifling impressions and cannot be matched back to the barrel through striations. Shotgun examination relies on a different evidence set.

Pellet pattern at the recovered distance is matched to test-fired patterns from candidate shotguns at known distances and with known chokes (cylinder, improved cylinder, modified, full). A tighter pattern means a tighter choke or a shorter range; the candidate weapon must reproduce the questioned pattern within tolerance.

Wad and shot cup recovery is often the single best identifying evidence in a shotgun case. The plastic shot cup and over-powder wad are stamped by the manufacturer and carry the gauge, the make and frequently a batch code. Recovered wads also pick up striations from the bore and ring marks from the choke constriction.

Slug examination borrows the rifled-barrel toolkit if the slug was fired from a rifled slug barrel or rifled choke; otherwise the examiner relies on bore-diameter, mass and metallurgy class characteristics. The book chapter on firearm injuries (entry, exit, range)covers what the same pellets do to tissue.

The Association of Firearm and Tool Mark Examiners (AFTE) Theory of Identification gives the examiner a fixed vocabulary for the final report, with each conclusion rung carrying a precise definition that can be tested in cross-examination.

1. Identification. Sufficient agreement of individual characteristics so that the practical likelihood of another weapon producing them is excluded. This is the only conclusion that says "same weapon".
2. Inconclusive A. Some agreement of individual characteristics and all class characteristics, but insufficient for identification.
3. Inconclusive B. Agreement of all class characteristics, no disagreement of individual characteristics.
4. Inconclusive C. Agreement of all class characteristics, with some disagreement of individual characteristics.
5. Elimination. Significant disagreement of class or individual characteristics. Different weapon.
6. Unsuitable. Item lacks sufficient detail to support any comparison.

Modern ballistics labs front the manual comparison with an automated 3D imaging and correlation database. The two examiners-testable names are IBIS(Integrated Ballistic Identification System, Forensic Technology Inc., Canada) and NIBIN(National Integrated Ballistic Information Network, run by US ATF). Both image the breech face and firing-pin region of a case and the bullet body in 3D, generate a numerical correlation score against the database, and produce a ranked candidate list that an examiner then confirms on the comparison microscope. India runs ballistics divisions at CFSL Chandigarh and CFSL Hyderabad under the Directorate of Forensic Science Services (DFSS), and the National Ballistic Data Centre concept under the Bureau of Police Research and Development (BPRD) has been piloted as the Indian equivalent of NIBIN. State SFSLs maintain their own test-fired reference collections for licensed weapons.

The expert report ends in court. Under the Bharatiya Sakshya Adhiniyam 2023, expert opinion on firearm matches is admissible under the equivalent of Section 39(formerly Section 45 IEA), and every recovery, transit and examination step has to be logged in the chain of custodyregister. The Daubert (US) and Frye (US) admissibility standards are frequently cited in comparative ballistics literature: Daubert asks whether the technique is testable, peer-reviewed, has a known error rate and is generally accepted; Frye asks only the last. Indian courts have not adopted either name, but the substance of Daubert (reliability + general acceptance) informs how trial courts weigh ballistic expert evidence.