Ballistic is by far the best shooting app for an Ipad.
Here is a link to a thread posted last week asking the same question. I am sure there are more threads discussing the topic, but you will have to look around.
Have Ballistic AE, Shooter and Bullet Flight. Of the three Ballistic is my favorite. Have it on my iPhone as well and use it at the range. Pretty much dead on if you have your velocities properly clocked. Another neat application is MD Ballistics for Mildot calculations.