coincidental rather than reflecting a shared source will be very small… Forensic individualization sciences that lack actual data, which is most of them, have no choice but to either intuitively estimate those underlying probabilities and calculate the coincidental match probability from those subjective probabilities, or simply to assume the conclusion of a miniscule probability of a coincidental match (and in fact they do the latter).
In the specific context of firearms and toolmark examination, derivation of an objective, statistical basis for rendering decisions is hampered by the fundamentally random nature of parts of the firing process. The exact same conditions—of ammunition, of wear and cleanliness of firearms parts, of burning of propellant particles and the resulting gas pressure, and so forth—do not necessarily apply for every shot from the same gun. Ultimately, as firearms identification is currently practiced, an examiner’s assessment of the quality and quantity of resulting toolmarks and the decision of what does or does not constitute a match comes down to a subjective determination based on intuition and experience. By comparison, DNA analysis is practically unique among forensic science specialties as having a strong objective basis for determination and as being amenable to formal probability statements.
Thornton and Peterson (2002:Fig. 1) rank various forensic science subfields on a continuum of relative subjectivity. On the low end of that scale is DNA analysis, along with serology (blood type determination) and drug and narcotic identification. They identify firearms and toolmark identification as having relatively high subjectivity, on par with fiber identification. They identify blood spatter interpretation, voiceprint analysis, and bite-marks as a group of forensic science specialties just slightly more subjective than toolmark identification, and handwriting and hair identification as a cluster slightly more subjective yet.
Smith (2004:130) succinctly summarized the basic task of a firearms examiner in making an identification between pieces of evidence:
Before a microscopic comparison begins, a foundation is built by measuring and comparing available class characteristics, such as General Rifling Characteristics (GRCs). These objective criteria are used to narrow the pool of candidates for determining a common source. Once an available foundation has been established, a common source often can be determined by evaluating individual microscopic marks of value using pattern recognition.
In traditional firearms identification—part science and part art form, still carried out today using the same basic tools that gave rise to the field