Check out AR-15 zeroing.com for great instructions for bore sighting an AR15 assault rifle (and its M16A2 and M4 Carbine cousins).

Pre-sighting AR-15 sights: 

• Bore sighting for zero at 50 yards should give you a relatively flat trajectory at 200 yards.  This means that the bullet crosses your actual line-of-sight twice, once at 50 yards and again at 200 yards with a 7 inch lift in-between.
  
  
•  Bore sighing at zero at 25 yards should give you a relatively flat trajectory at 300 yards but will be high at 100 and 200 yards.
  
  
•  The US Marines calls their procedure “Battle site Zero”, and they bore sight for zero at 36 yards 
  Battle sight Zero settings (flat trajectory at 300 meters (328 yards)
  

 Pre-sighting the AR-15 iron sights on the Colt Mark Target

On my Colt AR-15, the “z” mark can be found 2 clicks above the “6/3” setting on the rear sight elevation knob.

You must adjust your rear-sight elevation to this setting and use the small rear sight aperture (small peep).

With this setting, you laser sight the Colt AR-15 rifle to 25 meters (82 yards).

This achieves a flat trajectory at 328 yards (the 300 meter battle sight zero).

Also see my notes on Bolt Action vs. semi-automatic .223 rifles.