Smitho is correct to a degree. That's why they call it "somkeless powder"
There's still some smoke visible depending on barrel length and lighting conditions with modern powders. I tend to notice it much more indoors than outside.
Old weapons tend to need some pretty liberal lubrication to work reliably so I don't know how I feel about the "too much oil" thing.
Even modern arms will curl out a bit of smoke after some rapid fire though it generally comes from the breech and not the muzzle.
I'm currently only using smoke decals on suppressed weapons because I'm using no flash effect and it looks pretty lame without some sort of effect. I have it hooked up on assault rifles but am going to tone it way down. When I get around to shotguns, I'll use smoke there as well (I like to shoot cheap S&B brand buckshot and it generates some serious pollution lol)