When the light is right. Most often seen when sun is low behind the shooter, and jacketed ammo is used, and the range is more than a few yards. It's REALLY fun shooting .44 or .45 at a hundred or 200 yards. This is most often seen by some observer other than the shooter, who, of course, is probably concentrating on the sights. Less commonly, I've even seen the same thing with rifle bullets.
Think I read somewhere that you can see almost every round if you can arrange to shoot through drifting smoke, in dim light, with car headlights to your rear. Never thought it was important enough to set up that situation, though.
Best,
Johnny