Downunder (i.e. in Australia and New Zealand), you can say "James is a gun at tennis" meaning "James is a tennis master".  People have conversations like this:

My intention with this website is to help people become guns at Japanese.  I don't want people settling for "conversational" Japanese or Japanese "proficiency" (whatever that is).  I want users of this site to become generally acknowledged guns at the language.

Like a bullet whizzing past your nose, people will hear your Japanese and go, "Wow."