Well, assuming you haven't tried it, the best way to do anything that's involved with gravity and so on is to use "yVel"- basically, Y Velocity, or Vertical Velocity.
When the bullet is fired, you set its yVel to whatever it is you want it as (I'm assuming in this case it'll be determined by how much energy they put into the shot). Let's say -5 for now. Then, at the end of the loop, or wherever you adjust the bullet sprites and so on, you change the sprite's Y position by adding on its yVel. Then, you add 1 onto its yVel, and so every time it gradually rises less and then eventually falls.
Hope that's explained properly. :P