Chapter 15 — Key Takeaways

A scannable one-page card. For the full argument and the worked figures, see index.md.

The core claims

• "Ballistics" has two meanings — keep them separate. Strictly, ballistics is the physics of projectiles (internal / external / terminal). In court it loosely names the whole discipline of firearms examination, including striation comparison — which is not really ballistics at all. The conflation is the first small inaccuracy television teaches.
• Three phases, three jobs. Internal ballistics is where the marks are created (and where malfunctions live). External can constrain where a shot came from — but cannot survive an undetected ricochet. Terminal characterizes the wound (range, direction, energy, with the pathologist) — but rarely names the specific weapon and is almost silent on who fired.
• Rifling marks a bullet in two layers — and the whole validity debate is telling them apart. Lands (the high parts) and grooves (the cut channels) spin the bullet for accuracy and, as a byproduct, engrave it. Class characteristics (caliber, number of lands/grooves, twist direction) exclude and narrow but never individualize. Individual characteristics — fine striations from a barrel's microscopic wear — are real as a phenomenon, but the examiner's ability to read them reliably enough to individualize is what has never been adequately measured.
• Beware the third category: subclass. Marks shared by a batch of barrels cut with the same imperfect tool look like individuality and produce some of the worst firearms errors.
• A cartridge case is often a richer witness than a bullet. The cartridge case records four or five contacts — firing-pin, breech-face (usually the richest), extractor, ejector — and never deforms downrange. Their positions and shapes reliably tell you the type of firearm and can exclude a candidate cleanly; only the fine texture argues for the individual gun. Case-to-case grouping ("same gun") needs no gun at all and is often the strongest, most defensible firearms statement.
• The "match" is a subjective judgment. Comparison microscopy + the AFTE "sufficient agreement" standard = the examiner's experienced opinion that two surfaces look alike beyond the best different-source agreement they have ever seen. There is no fixed threshold, no count of striations, no statistic. It can be a good opinion — but it is an opinion, long dressed as objective certainty.
• NIBIN/IBIS generates leads, not matches. The database images and correlates casing marks and returns a ranked list of candidates for a human to test. Every lead must be confirmed by an examiner at the comparison microscope. "The database matched it" is the central error. Its real power: letting unrelated scenes find each other across time and jurisdiction.

The method-validity verdict (NAS 2009 / PCAST 2016)

Where it sits: above the discredited methods (bite marks, Ch. 16) — barrels and breech faces really do differ and the class characteristics are validated — but below DNA and instrumental chemistry, because the central individualization claim rests on a subjective standard without a quantified error rate. Contested is the fair word. Courts are split (e.g., United States v. Glynn restricted the language; others admit it unrestricted).

What you can honestly say on the stand

• Exclusion: "The recovered bullet has six lands and grooves with a right twist; the suspect's revolver has five with a left twist. The suspect's revolver did not fire this bullet." (Clean exclusion.)
• Same-source grouping: "These five cartridge cases were all fired by the same firearm." (No gun needed; strong.)
• Identification, honestly framed: "I found agreement in the class characteristics and in the individual marks, exceeding what I have seen between different-source samples, and in my opinion the recovered firearm is the source. There is no validated random-match statistic for these marks, and I am stating an opinion, not a certainty."
• NIBIN: "The database returned this firearm as a candidate; that is an investigative lead, which I then confirmed (or could not confirm) by independent comparison."
• What you must NOT say: "to the exclusion of all other firearms"; "reasonable degree of scientific certainty"; "100% match"; or any conversion of a class-consistency into an individualization, or of a database lead into a finding.

Key terms (one line each)

• Ballistics — strictly, the physics of projectiles (internal/external/terminal); loosely, the whole firearms-examination discipline.
• Rifling — the spiral lands and grooves in a barrel that spin and stabilize the bullet and, in the process, engrave class and individual marks onto it.
• Lands and grooves — the raised metal (lands) and cut channels (grooves) of rifling; their number, width, and twist are class characteristics readable from a fired bullet.
• Striations — the fine parallel lines a barrel's microscopic irregularities scrape onto a fired bullet; their correspondence is the contested basis of a same-source identification.
• NIBIN/IBIS — the National Integrated Ballistic Information Network and its Integrated Ballistics Identification System imaging engine; correlates casing/bullet marks to generate investigative leads, not matches.
• Cartridge case — the metal cylinder holding primer, powder, and bullet, ejected when a self-loading firearm fires; records firing-pin, breech-face, extractor, and ejector marks, and is often a richer witness than the bullet.

The cold-case line

The stray 9 mm cartridge case from the cabin runs through NIBIN/IBIS and returns only low-confidence leads — but the autopsy (Chapter 11) shows no gunshot wound. Terminal ballistics has nothing to read. The casing is a probable red herring: present at the scene, inconsistent with the manner of death, implicating and excluding no one. Log it fully (you don't yet know it's irrelevant), then say so plainly. Status: cartridge unrelated; teaches relevance. The exclusion/inclusion status of Keller, Whitfield, Salas, and Renner is unchanged.

The themes this chapter advanced

• Exclusion over proof (and recognizing irrelevance) — class characteristics exclude cleanly but rarely individualize; the cold-case casing's most important property is that, against the autopsy, it is irrelevant. Refusing to be led by it is itself a forensic skill.
• The validity spectrum — firearms ID is real and useful but sits in the contested middle: validated at the class level, subjective and unquantified at the individualization level; explicitly below DNA and above the discredited bite mark.
• (Also touched: cognitive bias — the subjective "sufficient agreement" decision, made while knowing the wanted answer or a database lead, is a near-laboratory model of bias, §15.4; and the CSI effect — the television "ballistics match" that means "certainty," §15.4 and §15.6.)