Whoops, I totally forgot to login...damn it's way too early/late...
Explosives detectors discriminate against gunpowder/residue. This is for a majority of reasons, a large one being, lots of people own guns, and shoot guns, and dealing with the paperwork of a positive hit for every person going to the shooting range would be expensive and man-power consuming.
I know this because I've asked our guards. I work at a facility (not telling what ;) that has GE brand explosives detectors before the x-ray and metal detectors. The guards still have to go through them, AR-15s, 9mms, ammo, and all. If the detectors didn't discriminate against munitions residue, no guards would be able to clear it.
And no, they don't all fire their weapons all the time, so they don't always have residue on them. But they do regularly go to the site shooting range and conduct training on site that uses blanks. It's like laser tag, with machine guns and blanks...I wish I could have a go at it... =D