That should have been done. I have not heard anything about that, so I don't know if it was or not.
The test checks for the presence of nitrates. It will not show positive if the gun was just handled. It would show positive if the body parts checked (normally the hands) were near a gun when it was fired. He would not necessarily have to be holding the gun when it was fired.
False positives are possible. For instance, if you had been handling or spreading fertilizer, your hands would then test positive for nitrates.
The residue can be washed off, but I am not sure how hard that is to do. It may take more than one washing.
Over time, skin sloughs away. So anything on the surface of the skin, whether soluble or not, will be shed in a few days.
Great explanation - but what I think could be important now, because of his changing of his testimony claiming he didn’t like guns (or use guns); if they had taken the swab of his hands and clothing, they could disprove his new statements.
That’s the point I was trying to make.