The repetitions of drawing and firing on a target, either dry or live, culminates in an unconscious implementation during a stressful situation. While it does get easier after numerous stressful events the trick is to make perfect (or as near perfect) reps as you can so that when the adrenaline dumps and you begin to act on auto-pilot it will be the very specific reps you did and not something else.
There are a myriad of ways to add stress to help inoculate someone from such issues, but stress training should only be attempted once a person has a meaningful mastery of such skill sets, or else it'll be counter-productive to their development.