I find that a bit hard to swallow...Especially with the diamond in the rough analogy.
The science of ballistics is very accurate and has been instrumental in bagging many a bad guy...
Ballistics are a quite reliable method of identifying a perpetrator, but ID evidence isn't available in many, if not most, cases because it needs both the gun and bullet and a way to connect the perp to both before such testimony is admissible in court. A traceable gun is one where both are available. That's why a .22 cal rimfire is an assassin's caliber of choice--the bullet is small so it bounces around before it exits (and, if the shot is to the head, it rarely exits) and the bullet can't be traced. Now, that's certainly an oversimplification because there are other uses for ballistic evidence (speed, trajectory, position, impact design, etc.).
Guns account for around 50% to 60% of murders a year. Traceable guns are available in a small percentage of those cases (the usual case is where the gun has been used in other crimes). Other weapons of choice, none of which are traceable, include cutting or stabbing with an object (knife, screwdriver, icicle, scissors, etc.), blunt object, strangulation, hands, fists, feet or pushing, arson, poison, drowning, explosives, narcotics, asphyxiation. Take a look at the latest FBI Uniform Crime Report. It's all in there.