The old canvas. Not really a 'gun' smell in fact, but nothing beats it.
Did ya ever have an old shelter half as a kid, sleep under one at Ft. Benning (in 1990! Amazed that something so old was still in your TA-50), sit in the back of the wet deuce and a half, and still sniff every time you open your garage, just to make sure--I don't want my kids to miss that smell!