Which of the following elements is NOT included in primer residue?

Primer residue is the material that is expelled from a firearm when a shot is fired, and it primarily consists of the components used to create the primer. The common elements found in primer residue include barium, antimony, and lead, all of which are utilized in the formulation of primer materials to initiate the combustion of gunpowder.

Barium is often included because it helps to improve the reliability of the primer. Antimony frequently appears in primer formulations as well, contributing to the efficiency of the firing mechanism. Lead, which has been a traditional element in primers, has also been a significant component, although there is a shift away from lead-based primers due to health concerns.

On the other hand, dumetallium is not a standard component of primer residue. This material does not relate to the chemical makeup of the primer itself and is not typically found in firearm discharge residues. Therefore, the absence of dumetallium from primer residue makes it the correct answer to the question.