Lee Lofland (founder of Writers' Police Academy) would like to point out yet again, there is no such thing as cordite, people!
Although your favorite TV detective might glean that a vic was shot very recently because the smell of cordite is lingering in the air, which also means the perp can't have gone far...no, he/she doesn't, seeing as how they haven't manufactured the stuff since World War II. I don't know why this is so prevalent after all these years, but it is. For some reason, readers/viewers with little to no knowledge of police work or detective work have been conditioned to understand that the supposed presence of cordite = a gun has very recently been fired.
Nope. And I keep hearing real cops don't use "perp", either.
One of the panelists at the L.A. Times Festival of Books mentioned that if you get gun details wrong, you'll hear about it from numerous readers. I would start by not using cordite as a story device.