Buck fever
How to control buck fever?? Same way an experienced actor deals with stage fright. It's there; it won't go away, you just don't let it control you.
Breathe slow, concentrate on the task at hand (sight picture or your entrance cue) don't think about all the horrible things that could go wrong.
Practice, practice, practice, beforehand. If you don't know your lines, backwards, forwards, and upside down, you have no one to blame but yourself when you forget them onstage. If you haven't been to the range, shooting, shooting, shooting, until sight picture and trigger squeeeeze are just a part of your being, well, that Jordan Buck is going to flip you the hoof and prance off into the distance.
Sorry, but there is no royal road to competence. You just have to put in the practice time.