Not really gun parts but....
One fine day wife and I popped for RDX/HMX on a chemical swab. Multiple times (cross contamination is a beotch). Fortunately, we'd had the foresight to bring our IDs and certifications sheets with us (we both work in the explosives industry). While not get out of jail free cards, the TSA was a lot more willing to cut us some slack after hand searches of every nook and cranny of our bags yielded nothing.
Took us about 1.5 hours to get through security, but we still made our flight.