If I could become world class at something, I can think of a ton of stuff I want to be an expert on. I would have to say the best choice would be trust. Whatever happens in life, I'd have no stress, I'd just be able to trust that everything will be ok and make logical decisions calmly.

As for the gun, it depends if I have achieved trust yet or not. For right now, a honest answer would be; I'd likely freak out, and freeze on the spot, blood pumping, with crazy thoughts racing through my head.