What is a common challenge when assessing gunshot residues?

Environmental contamination is indeed a common challenge when assessing gunshot residues (GSR). This challenge arises because residues can be inadvertently introduced from various sources, such as dust, soil, and even other people's clothing. Such contamination can compromise the accuracy of results, leading to potential false positives or negatives. For instance, if a sample shows GSR when it actually came from an external source, it could lead investigators astray in their analysis. Conversely, if the GSR is present but is masked by other contaminants, it may be overlooked, affecting the outcome of an investigation.

The other choices highlight important aspects related to gunshot residue, but they do not capture the central challenge as effectively. While extensive training is indeed necessary for proficiency in GSR analysis, it is not as pervasive a challenge as contamination. Similarly, while gunshot residues can evaporate, the primary issue lies in the introduction of foreign materials affecting the testing process. Lastly, the idea that only specific types of firearms leave residues is misleading; many firearms can produce GSR, which complicates the analysis rather than serving as a singular limitation. Thus, environmental contamination stands out as a significant issue in the assessment of gunshot residues.