Forensic Ballistics

What is forensic ballistics

Forensic ballistics is the application of ballistics — the physics of projectiles in motion — to firearm casework. The discipline asks four questions: what kind of weapon fired this projectile, did this specific weapon fire it, what trajectory did it take, and what was its terminal effect on the target. Each question maps onto a sub-branch.

The four traditional branches are:

• Internal ballistics — what happens inside the barrel between primer ignition and bullet exit. Pressure-time curves, propellant burn rate, barrel rifling.
• External ballistics — flight from muzzle to target. Drag, drop, wind drift, stabilising spin.
• Terminal ballistics — what the bullet does on impact. Penetration depth, expansion, energy transfer.
• Wound ballistics — the medical sub-branch. Permanent vs temporary cavity, secondary projectile effects.

A casework forensic ballistician spends most of their bench time on a fifth branch — comparative ballistics — matching recovered bullets and cartridge cases to a specific firearm via class and individual characteristics on the projectile and brass.

Internal ballistics

Once the primer fires, the propellant ignites and burns. Burning rate is pressure-dependent — the higher the pressure, the faster the burn, the higher the pressure, in a self-reinforcing loop until the bullet leaves the barrel and the system depressurises. Peak chamber pressure for a standard 9mm Parabellum sits around 235 MPa; for a 5.56×45mm NATO rifle round it is around 380 MPa.

Key internal-ballistics parameters:

• Peak chamber pressure — set by load, propellant choice and barrel geometry. Forensic relevance: pressure-induced primer flow signatures help identify the weapon class.
• Muzzle velocity — the bullet's speed at the moment of barrel exit. Driven by propellant type + load + barrel length.
• Rifling — helical grooves cut into the barrel that impart spin to the bullet for gyroscopic stabilisation. Number of grooves, twist direction (right or left), twist rate (e.g. 1:7 inch), and groove depth are all class characteristics.

The rifling imprints on the soft jacket of the bullet as it traverses the barrel — these are the land and groove impressions that comparative microscopy reads.

External ballistics

In flight the bullet experiences gravity (constant downward acceleration), drag (proportional to v² to first order), and crosswind. Drag is the dominant effect after gravity for short ranges and dominates absolutely beyond ~300m for a typical handgun round.

The trajectory is a slightly-flattened parabola superimposed with an exponential velocity decay. For exam-level questions the time-of-flight and drop calculations reduce to standard kinematics:

• Drop in time t (no drag): h = (1/2) g t²
• Time-of-flight at range R (small angles): t ≈ R / v₀
• Combining: drop ≈ (1/2) g (R / v₀)² — the basis of "minute of angle" sight adjustments

The bullet drop tables that snipers and casework reconstructionists use are integrations of these equations with empirical drag coefficients for each projectile shape (G1, G7 reference profiles).

Terminal ballistics

When the bullet meets a target, kinetic energy converts into work done on the target: deformation of the bullet, deformation of the target, heat. The transfer rate depends on bullet construction (full metal jacket vs hollow point vs frangible) and target density (soft tissue vs ballistic gel vs bone vs hard armour).

Two cavity types in soft-tissue penetration:

• Permanent cavity — the literal hole the bullet leaves. Set by bullet diameter and yaw / fragmentation behaviour during transit.
• Temporary cavity — the radial expansion of tissue during the microsecond after passage, driven by hydrostatic shock. Returns to near-original geometry but causes secondary tissue tearing in dense organs.

Wound severity correlates with energy transfer per centimetre of penetration, not raw kinetic energy. A small high-velocity rifle round with rapid yaw delivers more wound trauma than a heavy slow handgun round at the same total energy.

Comparative microscopy

The forensic-ballistician's signature task. A comparison microscope (split-field optics with a single eyepiece showing two specimens side by side) compares striae on bullets and breechface marks on cartridge cases.

Class characteristics identify the weapon type:

• Calibre, rifling twist direction, number of lands and grooves, groove depth ratio (GRC)
• These narrow the candidate-weapon set to a class (e.g. "5.56 NATO, six lands and grooves, right-hand twist, 1:7" — points to AR-platform rifles)

Individual characteristics identify the specific weapon:

• Tool-mark striations from the rifling cutter, breechface, firing pin, ejector, extractor
• These are random manufacturing accidents that survive across the life of the weapon

A comparison match requires sufficient agreement in individual striations — quantified by counting consecutively matching striae. The Association of Firearm and Tool Mark Examiners (AFTE) range-of-conclusions framework formalises the ID / inconclusive / elimination scale.

Gunshot residue (GSR)

When a firearm discharges, primer compounds (lead styphnate, barium nitrate, antimony sulphide for traditional primers; varying for lead-free) and propellant residues are vapourised and expelled in a plume. GSR particles are spheroidal aggregates of these elements — the characteristic morphology and elemental composition is the forensic signature.

Detection methods:

• SEM-EDS (Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy) — gold standard. Particles ~1 µm carrying Pb + Ba + Sb in the right ratios are diagnostic.
• AAS / ICP-MS — bulk elemental analysis from a swab. Fast but loses morphological information.
• Colour tests — Greiss test for nitrites (presumptive only), modified Griess for distance estimation from a target.

GSR persistence on hands is hours, not days — washing, sweating and contact transfer all degrade the deposit quickly. Persistence on clothing is longer (days to weeks).

Wound ballistics for the casework reconstructionist

Wound channel reconstruction overlaps with forensic pathology. The forensic-ballistician's contribution: estimating shooting distance from residue patterns, and bullet trajectory from entry / exit wounds plus intermediate ricochet evidence.

• Contact range: muzzle imprint, soot tattooing, stellate tearing.
• Close range (a few centimetres to ~60 cm): GSR pattern around entry, partial powder tattooing.
• Intermediate range (~60 cm to ~1 m): visible powder stippling without soot.
• Distant range: no visible residue at the entry — distance estimation requires GSR analytical confirmation on swabs from the wound margin.

Indian context — laws and casework standards

Indian firearms casework operates under:

• The Arms Act 1959 and Arms Rules 2016 — regulate possession, manufacture, and use of firearms. Categories (PB / NPB) determine licensing strictness.
• The Indian Penal Code §307, §302, §304 — attempt to murder / murder / culpable homicide — most casework projectile recoveries arise from these.
• CFSL / state FSL ballistics divisions — the only government bodies that issue casework ballistics certificates. Private labs are accredited under NABL ISO/IEC 17025 but rarely cited in court.

For the FACT exam the questions on ballistics tend to be class-vs-individual recognition, GSR pairing (analytical method ↔ what it detects), and basic external-ballistics calculations. UGC-NET adds case-law and instrumentation theory.

How to study ballistics

1. Build the four-branch mental map (internal / external / terminal / wound) — one A4 sheet, definitions only.
2. Memorise the comparative-microscopy class-vs-individual taxonomy. Worth 2–3 marks per paper, every paper.
3. Drill GSR analytical pairings — SEM-EDS for particle morphology, AAS for bulk elements, Greiss for nitrites.
4. Practice external-ballistics drop calculations under timed conditions. They are reliable mark-getters.
5. Read the Arms Act + IPC sections that frame Indian ballistic casework — at least once, in full.

A good reference: Hatcher's Notebook (Major General Julian Hatcher) for the classical theory; Heard's Handbook of Firearms and Ballistics for analytical workflows; the FBI's Ballistic Imaging report for the current state of comparative imaging databases (NIBIN).