The AR-15 rifle has two adjustable sights- front and rear. Zeroing elevation adjustments are made using the front sight, and windage adjustments with the rear.
The rear sight has an elevation knob with range indicators from 300 to 800 meters and two apertures for range. One aperture is marked 0-2 for short range from 0-200 meters and the unmarked aperture for normal range from 300 to 800 meters. This unmarked aperture is used in conjunction with the elevation knob for 300-800 meter targets.
The rear sight also consists of a windage knob on the right side of the sight. Each click of the windage knob will move the strike of the round from 1/8 inch at 25 meters to 4 inches at 800 meters. A windage knob pointer is on the windage knob.
The front sight consists of a rotating sight post with a spring loaded detent. This detent can be depressed using a sight adjustment tool, or using a sharp point such as a cartridge tip.
Establish Mechanical Zero -
Carefully aim and fire each shot of a 3-shot group at a paper target set up at 25 meters. If your shots are not striking the point-of-aim, then adjust your sights.
Continue to fire 3-shot groups and adjust the sights until you have a tight group at the point of aim.
Once this is done, the unmarked aperture is automatically zeroed. Rifle is now combat-zeroed all other ranges on the elevation knob are also zeroed, so to engage target at say 500 meters, turn wheel to 5.