This is a very good move to learn especially if you can get your self into a habit of using it.

I have got it to work with the battle rifle and carbine alike. To do the one hit kill you tap B+R(right trigger)+X. With your battle rifle you should lunge forward and hit your opponent, the X button will then cancel the rest of the lung so you will quickly pull your rifle down and then shoot (you need to be aiming at their head). This will happen so fast it is like one hit but I reality it is a beat down and a shot.