Chapter 15 — Exercises

Work these without looking back at the chapter first; then check yourself. Items marked † have full worked solutions in the answers appendix. There are no answers in this file. Mix of recall, applied reasoning, evidence interpretation, "spot the overstatement," ethics, and a cold-case extension.

A. Recall and definitions

1. Define ballistics in the strict sense, and then explain how "ballistics" is used loosely in the courtroom. Which part of this chapter — striation comparison or trajectory work — is not really ballistics at all?

2. Name the three phases of ballistics by where the projectile is, and state in one phrase the kind of forensic question each can answer.

3. † Define rifling, and explain the difference between the lands and the grooves. Which are the raised parts and which are the cut channels?

4. What is the aerodynamic purpose of rifling? Use the football analogy from §15.2 in your own words, and then state why its forensic usefulness is "an accident of its design."

5. Distinguish class characteristics from individual characteristics as they appear on a fired bullet. Give two examples of each.

6. Define striations as used in firearms (and toolmark) examination, and state what is supposed to create them in a barrel.

7. Define cartridge case, and list the four or five distinct marks a fired case can record. Why is a case left behind by a semi-automatic but retained by a revolver?

8. † What is NIBIN/IBIS? Expand both acronyms, and state in one sentence the single most important thing the system does not do.

9. Define subclass characteristics, and explain how they differ from both class and individual characteristics. Why are they a "live failure mode" in firearms work rather than a curiosity?

10. What is the breech face, and why is the impression it leaves on a case head often richer than the striations on a bullet?

B. Applied reasoning

11. A recovered bullet shows six lands and grooves with a right-hand twist; the suspect's pistol has a barrel cut with six lands and grooves, right-hand twist, in the same caliber. Has the suspect's gun been identified as the source? State exactly what has and has not been established, using the chapter's verbs.

12. † A recovered bullet shows five lands and grooves with a left-hand twist; the suspect's revolver has six with a right-hand twist. What can you conclude, and is it an exclusion, a consistency, or an identification? Why is this the strongest kind of firearms conclusion?

13. Explain why an examiner test-fires a suspect weapon into a water tank or gel block rather than into open air or a backstop. What is the test-fired bullet for?

14. Three cartridge cases are recovered from a scene and a fourth from a second, unrelated-seeming scene; all four are compared case-to-case (no gun is available). The examiner concludes all four were fired by the same firearm. Explain why this conclusion can be defensible even without the gun, and why it is often stronger than "this case was fired by the defendant's gun."

15. A bullet recovered from a brick wall is badly deformed, with much of its bearing surface flattened. An eager analyst proposes to "read the surviving striations carefully and call a match if enough line up." Identify two distinct problems with this plan, drawing on §15.2.

16. † A casing's marks are entered into NIBIN/IBIS, which returns a high-ranked candidate linking it to a pistol recovered in another county. A detective tells the press, "The database matched our casing to the gun." Identify two separate errors in that sentence and state what has actually been established.

17. Explain the difference between using a cartridge case's marks to determine the type of firearm and using them to identify the individual firearm. Which of these is on firm ground, and which is the contested claim?

18. Why can the distribution of ejected cases on the floor be a more reliable piece of reconstruction evidence than the "match" of any one of those cases to a gun? What scene-handling discipline does that make essential?

19. An examiner reports a comparison as "inconclusive." Explain two legitimate reasons a competent examiner might reach "inconclusive," and then explain the §15.4 worry about "inconclusive" being used as an escape hatch. How can over-use of "inconclusive" make a measured error rate look artificially low?

C. Evidence interpretation

20. Re-read Figure 15.1 ("A fired bullet's rifling impressions"). The figure notes that deformation has "obliterated part of the bearing surface, so some striations are simply missing." Explain why "missing" striations must not be read as "non-matching," and what error the opposite reading would produce.

21. † Re-read Figure 15.2 ("What a fired cartridge case records"). The block says the fine breech-face correspondence "may SUPPORT same-source." Write (a) the single sentence an honest examiner could say on the stand about this casing, and (b) a single sentence that would overstate it.

22. An examiner's report states that the recovered firearm "cannot be excluded as the source of the questioned cartridge case, and its class characteristics are consistent with having fired it." List three separate ways this sentence is appropriately honest (i.e., where it stays inside what the method supports).

23. A trajectory reconstruction is presented as a single line drawn to three decimal places of an angle, placing the shooter at one precise spot. Using the §15.1 At the Bench callout, name two questions you would ask before trusting it, and state what the honest output form of a trajectory analysis should be.

24. In the cold case, a single fired cartridge case is recovered from the cabin debris. State precisely what the autopsy finding does to the forensic relevance of that casing, and why logging it fully was nonetheless the correct decision at the time of collection.

D. Spot the overstatement / junk-science alert

25. A firearms examiner testifies: "This casing was fired by the defendant's pistol, to the exclusion of all other firearms, to a reasonable degree of scientific certainty." Identify each of the three problematic parts of that sentence and explain, for each, what the method has not actually shown. Then rewrite the conclusion honestly.

26. † A prosecutor's slide reads: "Ballistics: 100% match. This is the murder weapon." Name the specific overstatement and the implicit error rate being claimed, and contrast it with the PCAST 2016 finding about the discipline's foundational validity.

27. A detective argues, "The wound proves a .38 was used, and the defendant owns a .38, so the defendant's gun fired the fatal shot." Untangle the three separate inferential leaps in that sentence and say which phase of ballistics (§15.1) each belongs to — and where the chain actually breaks.

28. A television expert glances at a striation pattern and declares an instant, certain match with a triumphant click. Using §15.4, give two reasons this is backward from how a real comparison-microscope examination actually proceeds.

E. Ethics and reasoning

29. A firearms examiner is handed a casing together with a NIBIN lead naming the suspected gun and a note that "the detective is confident this is our shooter." Using the §15.4 Cognitive-Bias Watch (and previewing Chapter 31), explain how each of those three pieces of context can lower the threshold for perceiving "sufficient agreement," and describe the safeguard.

30. † A laboratory's procedure has a second examiner "verify" every identification — but the second examiner is told the first examiner's conclusion before looking. Explain why this "verification" can confirm a shared error rather than catch it, and state what a genuine independent verification would require.

31. The "sufficient agreement" standard is defined, in the AFTE framework, partly in terms of the best agreement the examiner has ever personally seen between different-source samples. Explain why a standard that rests on an individual examiner's accumulated experience is difficult to present to a jury as "objective science," even when the examiner is highly skilled and honest.

32. You are asked to testify that firearms identification "individualizes a weapon the way DNA individualizes a person." Explain why you should decline, what specifically separates the two methods on the validity spectrum, and what you can honestly say instead.

F. Synthesis and validity spectrum

33. † Place these four methods on the NAS 2009 / PCAST 2016 validity spectrum (strong → discredited), justifying each: single-source nuclear DNA (Chapter 7); firearms identification (this chapter); latent-fingerprint comparison (Chapter 14); bite-mark comparison (Chapter 16, previewed). State explicitly where firearms ID sits relative to fingerprints and why reasonable reviewers disagree about the order.

34. In Chapter 14 you met Brandon Mayfield, a fingerprint identification made with total confidence that was completely wrong. Name the single safeguard — the one most firearms units still lack — that would most reduce the analogous risk in a firearms identification, and explain why it attacks the actual mechanism of the error.

35. In one paragraph, explain how this chapter advances at least two of the book's four themes (exclusion over proof; the validity spectrum; cognitive bias; the CSI effect cutting both ways). Name which themes and how.

36. Distinguish, with a one-line example of each, the four AFTE-framework conclusion categories used in this chapter: identification, elimination, and inconclusive (and note which of the first two this book most prizes, and why).

G. Cold-case extension

37. † Cold Case. Using only what the stray cartridge case and the autopsy establish, write the entry you would add to the Mill Creek evidence log (Appendix I) for this chapter. State (a) the defensible status of the casing at its true strength, (b) the honest verb, (c) at least three things this item specifically does not establish, and (d) why the exclusion/inclusion status of Keller, Whitfield, Salas, and Renner is unchanged by it.

38. Cold Case extension. A junior investigator wants to chase the casing's low-confidence NIBIN correlations to "unrelated incidents elsewhere in the state," reasoning that any lead is worth pursuing. Using the chapter's lesson on relevance, argue for or against spending limited lab resources on this, and explain what the autopsy finding contributes to that decision.

39. Cold Case, integrative. Explain how the casing in this chapter illustrates theme one of the book ("forensic evidence excludes — and recognizes irrelevance — far more reliably than it proves") better than a dramatic "match" would have. Why is recognizing an irrelevant item "as much a forensic skill as any comparison"?

H. Short writing

40. In 150–200 words, explain to a juror the difference between what a firearms examiner may honestly say — "the class characteristics are consistent with the recovered gun, and in my opinion, exceeding what I have seen between different-source samples, it is the source" — and the television "ballistics match." Make clear why one is evidence to be weighed and the other is a claim the science has not earned.