Gunpowder residue is most likely to show up where on a shooters hands

Gunpowder residue is most likely to show up in the space between the thumb and the index finger on a shooter hands.

Explanation:

Gunpowder residues are the microscopic particles and other residual materials expelled from a firearm with each shot. These particles can be deposited on the face, hands and clothing of a shooter, and on any surface near the landfill (including the human, animal, or object being targeted).