Instructions
Assign the Hidden Gun to either the left or right bumper in a Profile Set, which can be accessed on the "Customize" menu.
Lock onto your target with the left trigger. A lock symbol will appear over your target's head if this is done correctly.
Press and hold the bumper that you assigned the Hidden Gun to. Hold the button for 1.5 seconds. A white line will extend from your Hidden Gun to your target if you can make the shot.
Release the bumper when the Hidden Gun is fully focused to shoot your target for an automatic kill.