Let's clear up what kind of bullet Keith Urban is singing about in "Without You". First part of the lyrics are below:
"I loved you since the very first day
when I caught you looking my way.
I smiled and just knew it.
And up until you came along
no one ever heard my song.
Now it’s climbing with a bullet."
I'm not very eloquent so I'll try to explain this as best I can. Once upon a time there was a magazine called R & R (Radio & Records). It monitored major radio stations all over the country to see what songs they were playing. These reporting stations determined the chart toppers in various genres.
If an artist's or group's record (song) was doing well and had a lot of spins ("spins" means it's being played often) then it would have a bullet point next to it in the magazine. The bullet point means it's a HOT song on its way UP the chart.