Gallaxhar: [enters on a personal hovercraft] You must be terrified. Waking up in a strange place, wearing strange clothes, imprisoned by a strange being floating on a strange hovering device. Strange, isn’t it?
Susan Murphy: Hardly. It’s not the first time.
Gallaxhar: [pause, deflated] Wow, you really get around.

— Monsters Vs. Aliens

The more you think about a  situation and get used to the idea, the easier it is to deal with it. The more exposure you get to things that push your limits, the more those limits will expand. I’m not saying that training, practice, and even competing with a gun will fully prepare you for actually using it under stress, but it will at least get you thinking and hopefully ingrain good habits that will help. Normalization of the abnormal frees up your mind and body to do what needs doing instead of freezing at the sight of the unfamiliar.