Yes, you are exactly correct about being slightly out of focus causing the trail to be more visible. That's what we were taught when watching for them. They can be seen as long as the bullet is creating a supersonic shockwave, so as long as the projectile is going faster than sound, you get a vapor trail. In high crosswinds it is very hard to observe, and in really dry environments the trail is less visible, but a good spotter can trace it to the target. As to oil in the hollow point, I think you'd need an awfully lot for it to be visible, and the amount in one of the Hammer bullets wouldn't even be a full drop. It would be vaporized in about 10 feet, I think. And since its in the front of the bullet, I think it wouldn't make any difference, since it would be held there by momentum and pressure. That's why the Military puts tracer mixes in the back of a projectile instead of in the front.