I'm going to presume 2 things here first that you are asking how someone with an exotic handcannon killed you with one shot and second that you play on PlayStation 3 or 4. The exotic handcannon Hawkmoon has 2 abilities which combine to grant 3 bonus damage effects hereafter refered to as "procs" those being the "Holding Aces" and "One in the Chamber" abilities. These typically give the wielder 3 bullets randomly on any given reload which will damage the target more if the shot connects, however on rare occasion more than one proc can land on a single bullet and in an extremely rare chance all 3 can take effect on the same bullet. Odds are you were extremely unlucky and found yourself on the receiving end of one of these extremely rare super proc bullets.

???? I think you need to clarify what you mean. What you said doesn't make any sense