I visit the gun range on a regular basis. If I have a flight the next day, when/where should I tell TSA officers that I have been shooting at a gun range and therefore might have gun powder residue on my hands. I would assume it is better to tell them before you get into the security line so they know in the event I get selected for a hand swab while in line, but I'm not sure.
Some could say the way to handle this is the same way you would if you were traveling with a gun. In that case TSA recommends:
Declare each firearm each time you present it for transport as checked baggage.
That basically is akin to being proactive/preemptive. One could argue that announcing to TSA upfront you could have gunpowder residue means you're separated from the general group quicker and thus get your screening quicker and are on your way quicker to catch that plane with less hassle.
A contrary argument could be that there is no point being proactive/preempting something you're not definite will set off an alarm, so why bother and go through the hassle?
There is no literature on the TSA website which addresses the issue or says it is an offense not to inform TSA preemptively and hence the answer for this will have to be whichever option you are comfortable with.
Why do you think you need to tell them? Gun shot residue itself is not a prohibited item and I suspect is relatively common, especially at certain stations.
The worst case scenario is that if you get selected for random swabbing, it will trigger an alert sending you to additional screening. At that point, you can offer that you were shooting previously but it really doesn't matter. As long as no actual prohibited items (such as the gun ;) are found, you're good to go.