A person of whom preforms so well at their chosen activity and makes such an impact and with such apparent little effort, on their part, that it warrants a comparison to that of the a gun in a sword fight.
Criminal : I need a barrister to get me off this charge.
Mate: Have you heard of that guy in Canberra, they call him "the gun."
Criminal : Why do they call him "the gun?"
Mate: Because he gets everyone off
When someone calls the front right seat of a car, they shout out shotgun! But this one is for machine gun seat out on the back of the pickup truck. Ya know ww2, willys jeep.