Chapter 15 — Self-Check Quiz

24 questions: multiple choice and short answer. Try them closed-book. The answer key is in the collapsed block at the bottom.

Multiple choice

1. In the strict sense, ballistics is: - A. The comparison of striations on bullets under a microscope - B. The physics of projectiles in motion (how things fly when launched) - C. The study of gunshot wounds - D. The database of cartridge-case images

2. The phase of ballistics in which the forensically useful marks are created is: - A. External ballistics - B. Terminal ballistics - C. Internal (interior) ballistics - D. None — marks are created after the bullet lands

3. On a rifled barrel, the lands are: - A. The cut-away spiral channels (the low parts) - B. The raised metal between the grooves (the high parts) - C. The same thing as the breech face - D. Only present in shotguns

4. The primary purpose of rifling is to: - A. Leave identifying marks for forensic examiners - B. Spin and gyroscopically stabilize the bullet for accuracy - C. Slow the bullet down - D. Prevent the gun from jamming

5. Which of the following is a class characteristic of a fired bullet? - A. A unique pit from rust on one barrel - B. The number of lands and grooves and the twist direction - C. A random tool mark unique to one barrel - D. A scratch added by forceps during collection

6. Subclass characteristics are dangerous because they: - A. Are always easy to recognize - B. Are shared by a batch of barrels made with the same imperfect tool, yet look like individual marks - C. Only appear on cartridge cases, never bullets - D. Prove a same-source identification

7. A cartridge case is usually a richer comparison witness than a bullet because: - A. It is made of harder metal - B. It records four or five distinct contacts (firing pin, breech face, extractor, ejector) instead of one - C. It travels downrange and deforms - D. It carries a serial number

8. The impression on the case head that is often the richest single source of detail comes from the: - A. Extractor hook - B. Firing pin only - C. Breech face (the flat steel wall behind the chamber) - D. Rifling

9. The instrument used to compare a questioned bullet against a test-fired known, side by side in one field of view, is the: - A. Mass spectrometer - B. Comparison microscope - C. Scanning electron microscope - D. Refractometer

10. Under the AFTE framework, the possible conclusions of a firearms comparison are essentially: - A. Match / no match only - B. Identification / elimination / inconclusive - C. Guilty / not guilty - D. Class / individual / subclass

11. The historical standard for declaring an identification — "sufficient agreement" — rests fundamentally on: - A. A fixed, objective number of corresponding striations required by law - B. A validated random-match statistic, like DNA - C. The examiner's accumulated experience and subjective judgment - D. The serial number of the firearm

12. NIBIN/IBIS is best described as a system that: - A. Declares courtroom matches between casings and guns - B. Images and correlates cartridge-case marks to generate investigative leads for examiners to confirm - C. Reconstructs a bullet's trajectory - D. Measures the caliber of a bullet automatically

13. When IBIS returns a high-ranked candidate, what has been established? - A. That the two casings were fired by the same gun, conclusively - B. That a human examiner must still confirm or reject the link by comparison microscopy — the lead is a hypothesis, not a finding - C. That the suspect is guilty - D. That the case can go to a jury without further examination

14. The phrase most likely to draw a Daubert challenge and a hard cross-examination is: - A. "These four casings were fired by the same firearm" - B. "This casing's class characteristics are consistent with the recovered pistol" - C. "This casing was fired by this gun, to the exclusion of all other firearms" - D. "The comparison was inconclusive"

15. The 2016 PCAST report concluded that, at the time, firearms analysis: - A. Had clearly established foundational validity, like DNA - B. Fell short of demonstrated foundational validity, with too few well-designed studies and non-trivial measured error rates - C. Was as discredited as bite-mark comparison - D. Should be banned entirely from courts

16. Terminal ballistics in a death investigation can best establish: - A. The exact make and model of the firearm - B. Range, direction, and wound characteristics (shared with the pathologist) — but rarely which specific weapon - C. The shooter's identity - D. The serial number of the gun

17. A trajectory reconstruction should be reported as: - A. A single line to three decimal places of an angle - B. A precise pinpoint where the shooter stood - C. A region with stated uncertainty (often a cone), especially if deflection cannot be ruled out - D. A definitive map of the shooter's movements

18. Class characteristics on a fired bullet can: - A. Identify one specific gun to the exclusion of all others - B. Exclude a candidate weapon cleanly and narrow the field, but never individualize - C. Reveal the shooter's DNA - D. Prove when the shot was fired

Short answer

19. Explain, in two sentences, why "these five casings were all fired by the same gun" is often a stronger and more defensible statement than "this casing was fired by the defendant's gun."

20. Name three of the four documented sources of error in firearms identification listed in §15.4, and for each, state in one phrase how it can produce a wrong conclusion.

21. A bullet recovered from a wall has six lands and grooves with a right-hand twist; the suspect's revolver has five with a left-hand twist. State your conclusion and the honest verb for it.

22. A NIBIN lead is handed to the confirming examiner along with the information that the database flagged these two casings as probably the same. State the bias risk and the safeguard (preview of Chapter 31).

23. In the cold case, the autopsy (Chapter 11) found no gunshot wound on the body. Explain in two sentences why this single fact makes the stray cartridge case a probable red herring, and what the disciplined response to that item is.

24. Write one sentence a firearms examiner could honestly say on the stand about a strong cartridge-case correspondence, and one sentence that would overstate it.

Answer key (click to expand) **Multiple choice:** 1-B · 2-C · 3-B · 4-B · 5-B · 6-B · 7-B · 8-C · 9-B · 10-B · 11-C · 12-B · 13-B · 14-C · 15-B · 16-B · 17-C · 18-B **Short answer (model points):** **19.** Same-gun grouping of multiple casings is a case-to-case comparison that *avoids needing the gun at all* and avoids the deformation problems of bullets, so it rests only on whether the cases match each other. Naming the *defendant's* gun adds the contested individualization step — the leap from "same unknown source" to "this specific weapon to the exclusion of all others" — which is exactly the claim PCAST and the NAS found scientifically underbuilt. **20.** Any three: (a) **subclass characteristics** mistaken for individual ones — declaring same-source from marks actually shared across a manufacturing batch; (b) **confirmation/contextual bias** — knowing the wanted answer or the database lead lowers the threshold for perceiving "sufficient agreement"; (c) the **closed-set fallacy** — validation studies that force a pick from a known set overstate real-world accuracy where "none of them" must be allowed; (d) **inconclusive as an escape hatch** — classifying hard comparisons as inconclusive rather than risking an error can make measured error rates look artificially low. **21.** **Conclusion:** the suspect's revolver did **not** fire that bullet — the class characteristics (lands/grooves count and twist direction) are incompatible. **Honest verb:** *excludes* (a clean, defensible exclusion — the strongest kind of firearms conclusion). **22.** **Risk:** the lead is itself a source of *contextual bias* — the examiner now approaches the comparison already told by the machine that the two casings are probably the same, so the "verification" can become a rubber stamp rather than an independent test. **Safeguard:** do the confirming examination as blindly as practical (context management / sequential unmasking, Chapter 31), so the human is *testing* the lead, not confirming it. **23.** Terminal ballistics (§15.1) has *nothing to read* — no wound track, no recovered bullet from the body — so whatever the casing is, it is not the instrument of death; the body says unambiguously that Diallo was not shot. The disciplined response is to **log the casing fully** (photograph, NIBIN entry, class characteristics) because its irrelevance was not yet known at collection, and then record it honestly as a probable red herring that neither implicates nor excludes any person of interest — refusing to be led by it. **24.** **Honest:** "I found agreement in the class characteristics and in the fine breech-face detail, exceeding what I have seen between different-source samples, and in my opinion the recovered firearm is the source — stated as an opinion, with no measured random-match statistic available." **Overstated:** "This casing was fired by the defendant's pistol and no other firearm on Earth, to a 100% scientific certainty."