• automatic pistol bullets fired from revolvers;

  • pistol bullets fired from rifles by using adapters;

  • pistol bullets fired from rifles after being handloaded into rifle cartridge cases;

  • revolver-type bullets fired from automatic pistols;

  • replacement of barrel of an automatic pistol with another;

  • replacement of firing pin in automatic pistol;

  • refiling of breech face of automatic pistol or revolver;

  • refiling firing pin of automatic pistol or revolver;

  • replacing revolver barrel (which is more difficult than with more modular semiautomatics); and

  • relining a pistol or rifle barrel with a new rifled liner.

Other complications include “firing a bullet of one caliber through an arm chambered for a larger caliber” (“such a bullet will show sketchy and erratic rifling marks which will be of little help in establishing identification”) and “firing a pistol or revolver cartridge through a smooth bored barrel” (“the bullet will obviously show no rifling marks”).

In this section, we briefly review what has been discussed in the firearms identification literature about some of these challenging situations. It is not an exhaustive list but covers particular topics that are more germane to RBIDs containing images from new firearms.

Condition of Evidence: Damage, Corrosion, and Cleanliness

In forensic laboratories, firearms can be discharged into water tanks (or other nondestructive trap mechanisms) so that bullets may be recovered with no damage other than those left by the firing process; similarly, cartridge casings may be quickly and safely retrieved following controlled test firings. A basic, fundamental challenge of firearms identification is that crime scenes are not necessarily such controlled settings. The task of pattern matching between pieces of bullet or cartridge case evidence—considering which microscopically fine markings may be unique to the source firearm, filtering out other marks, and evaluating various possible alignments of the exhibits (including, for bullets, all possible rotations of the exhibit pairs)—is already a difficult one. But it can be made more difficult by the nonpristine nature of crime scene evidence. Bullets can strike wood, asphalt, human tissue and bone, or other substances and, consequently, can be seriously warped, deformed, or fragmented; they can be lodged such that even their recovery for analysis can cause damage. Likewise, cartridge casings expelled onto the ground may be crushed underfoot or exposed to the elements. The possible differences between pristine exhibits and crime-scene samples are a continuing challenge for conventional firearms identification and ballistic

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