Not that long ago my friend was shooting a .22 ruger and I was standing next to her while she was shooting. Kinda behind her, and at like a 40 degree angle. I was watching her pistol and for some reason was trying to see the muzzle flash or the smoke ( I never really get to see that when I fire, I guess it's because I blink after I shoot, even though my groups are good and I'm not really conscious of it.) That's when I saw what appeared to be really really really fast moving BBs or pellets. I couldn't see them every shot, but if I really looked for them, I could see like every 3rd or 4th shot. I guess it had alot to do with the fact that it was an indoor range with a brick wall to the side that added contrast, plus it was dark. I guess the bullets come out hot enough so that they glow a little, or leave a vapor trail. It's not painfully obvious, but you can see it. Same thing when she was shooting my .45 pistol. I couldn't see it with the 9mm though. Bullets going too fast?
Outdoors, and at long ranges, it's actually pretty easy to see vapor trails on rifle rounds if you are using a scope. If the humidity is high, you can sometimes see it with the naked eye. It works best if the shooter you are observing is shooting at far away targets, like 150+ yrds away, because the farther the bullet travels, the easier it is to see. At least that's what I've been able to observe.
It's pretty cool actually ^_^