Ballistics & Gunshot Residue: What the Physics and the Particle Can Support

Firearm and toolmark identification asks a question about a source: which gun fired this cartridge, which tool made this mark. Ballistics and gunshot residue ask a different kind of question, about an event. How far away was the shooter. What path did the bullet take. What did it do inside the body. Who was near a discharge. These are activity-level questions, closer to reconstructing a scene than to matching two marks, and the honest strength of an answer, and the way it fails, are different in kind.

A source claim overreaches when it names one gun to the exclusion of every other. An event claim overreaches in a quieter way: it takes a real, careful measurement and carries it past what it can bear, into a story about who did what and from where.

This reading follows the physics and then the transfer. First the wound and the block behind it: how far a terminal-ballistics reconstruction can honestly run backwards from an injury, and how much weight ballistic gelatin can carry as a stand-in for tissue. Then the residue: what a single particle actually means, how it moves onto people who never fired a shot, and how the conclusion should be worded so it reports the event and not the act. Throughout, the aim is the same as any honest testimony: state what the trace supports, state the assumptions, and stop there.

Reconstruction from a wound runs backwards, from the injury to the shot, and the physics bounds how far back it can go. Start with the tissue the bullet meets. Bone fractures at a striking velocity of only about 200 feet per second, and experimental work has perforated bone at speeds as low as 99 metres per second, near a reported human threshold around 61. Bone is easy to break, and once broken it becomes its own projectile: displaced bone spicules travel forward and cause damage further along the track. So the wound is not a clean recording of one object's passage. Some of what an examiner sees downstream was done by shattered bone, not by the bullet, and the two cannot be neatly separated.

Position is bounded the same way. Fired through a body's depth of tissue, a rifle bullet can deflect sharply: minimal deflection over the first few centimetres, but up to twenty-five or thirty degrees once it has crossed twenty-odd centimetres and begun to fragment, after which the direction becomes erratic. A careful reconstruction attaches an error cone of roughly thirteen degrees around the back-projected line, not a single ray to a spot on the floor. And the raw material is often thin: in one review, adequate wound descriptions were recorded in under three per cent of cases. None of this makes wound ballistics useless. It supports coarse, honest statements: that a wound is consistent with a larger rather than a smaller calibre, the direction of fire from bevelling, a trajectory over a short early stretch of track, and a reconstruction reported as a bracketed range. What it will not support is a precise calibre, or a precise place the shooter stood, offered to the jury as fact.

Behind many terminal-ballistics claims sits a block of ballistic gelatin, used as a stand-in for soft tissue. Its whole appeal is that it is standardised. The accepted calibration is specific: a 4.5 millimetre steel sphere fired at about 180 metres per second into ten per cent gelatin at four degrees should penetrate roughly 85 millimetres, the same depth reached in pig muscle. That single equivalence, gelatin to pork, is the entire empirical bridge from the block to living tissue. Within its own terms the medium is genuinely good: reproducible to a couple of millimetres across dozens of shots, transparent enough to film the passing bullet, and density-matched to muscle.

And the block is not a body. Gelatin is homogeneous; a body is skin over fat over muscle over bone, threaded with vessels and organs of different elasticity, and a bullet does not know it has left the gel. The validation, moreover, is against animal muscle: as recently as 2024 the field conceded there is no comparative data from human subjects. The feature juries find most vivid, the temporary cavity, is the least transferable of all, drawn by the method's founder as an approximation of a maximum and dependent on local anatomy; in one 2024 study two accepted methods measuring the same block disagreed with each other. The honest use is comparative. Gelatin can show that one load penetrates further than another under identical conditions, and it can exclude. Standardisation buys comparability, not a body, and a gel result should be offered as what it is: a calibrated comparison in a simulant, not a measurement of the wound in the deceased.

Gunshot residue analysis looks at micro-particles under a scanning electron microscope and sorts them by their elemental make-up. The distinction that decides most cases is between particles characteristic of a firearm discharge and particles merely consistent with one. The list treated as characteristic, coming only from primer, is short: essentially a lead-antimony-barium composition, and a couple of close relatives. Everything else points toward a firearm without excluding other origins. Classification rests on composition, on a permitted list of accompanying elements, and on the particle's molten, spheroidal shape.

Two further facts keep the honest examiner cautious. First, casework rests on almost nothing: in most positive samples only a handful of particles are found, and in a large share only a single particle, and when only a few are present the microscope search itself misses them a substantial fraction of the time. Second, the true characteristic particle is genuinely rare in the general population, which sounds reassuring until transfer is added: a person who only stood nearby, or shook a hand, or picked the gun up afterwards can carry the same particles. The defensible conclusion is a statement about the particle, not the person: residue of a composition and morphology characteristic of primer discharge was present, and classified accordingly. The leap to this person fired a gun is not something a particle can support.

The single most important fact about gunshot residue in a courtroom is that it moves. Presence on a person's hands does not establish that the person fired, because residue transfers from surface to surface and hand to hand. The clearest data come from the police themselves. In one Australian study, firearms-carrying officers carried characteristic residue at about eight per cent, against roughly a third of one per cent in the general population, and three-quarters carried at least a particle consistent with it. Simply receiving a firearm at the start of a shift left two-thirds of officers with at least one particle, and some with more than a hundred. In a staged arrest, a median of about a quarter of an officer's particles transferred to the person being restrained in a few minutes of contact.

Lead-free ammunition removes the classic signature altogether. Non-toxic primers use titanium and zinc rather than lead, antimony and barium, so the particles the test hunts for may simply be absent, and those metals have many more everyday sources. Where lead-antimony-barium particles do appear after a lead-free round, they can be old residue left in the barrel by earlier conventional firing, a memory effect rather than proof of this shot. The fair counter, which an honest examiner should also give, is that transfer is not unmanageable: with control sampling of the arresting officers, gloves, one suspect per officer, and, in some jurisdictions, tagged police ammunition, contamination can be detected and bounded. The danger is greatest precisely where the count is small and the arrest was ordinary, and that is exactly the case in which consistent with having discharged a firearm should not be said.

How a ballistics or residue conclusion is worded is where the overclaim lives, and the fault line has a name. A gunshot-residue positive is a source-level finding: it answers whether the particles are residue. The question the court cares about is activity-level: did this person discharge a firearm. The discipline's standard framework was built to answer the first question, and reaching the second requires transfer, persistence and background contamination to be weighed under competing accounts of the event. That step is often skipped. In one survey, a substantial share of laboratories would report a single characteristic particle as a positive, with none of that evaluation attached.

There is a documented cost to getting this wrong. Gunshot residue has featured in wrongful convictions, and the wording consistent with having discharged a firearm has done work in front of juries that the science cannot support. The honest report states what the residue can carry: that a person was in the environment of a discharge, or in contact with a fired weapon or a contaminated surface, expressed as a likelihood ratio under stated, competing propositions. The same discipline governs the physics. A wound or a gelatin result supports a bounded, error-bracketed reconstruction, not a precise account of range, calibre and position. In both wings the rule is the same: report the event the measurement can support, name the assumptions, and do not let precision of measurement stand in for proof of the act.