not sure if the challenger has a trigger adjustment, but this is the procedure for a Medalist
"The adjusting screw to regulate the trigger pull is located on the rear face of the frame. Turning the screw in a counter-clockwise direction lightens the trigger pull; turning it clockwise increases it. The adjusting screw to regulate trigger backlash is located in the top surface of the trigger. Turning it clockwise decreases backlash.
The micrometer rear sight is click-adjustable for both horizontal and vertical correction. Each click, horizontal or vertical, equals one minute of angle."