Zeroing Your Rifle - Sight Relationships - MOA Minute of Angle - More Information

External Ballistics are used to describe the behavior of a bullet after it leaves the muzzle of a gun until it reaches the target.  The following diagram is useful to help understand how bullets actually fly in a long arc, before gradually slowing and falling back to earth.

Since the sights on a gun are actually above the barrel, one can assume the barrel is actually tilted up slightly in order for the bullet to hit where the sights are pointed.  The height of the sights above the barrel will vary from gun to gun.  The M16 for example probably has some of the highest sights of any common small arm.  The exagerated height of the M16 sighting system permits the shooter to utilize the sights more easily than on a traditional rifle since he or she does not have to lower his head and bend his neck as much to view down the sights.  This extra height however makes it more challenging to understand the zeroing of the M16.  In practice, the U.S. Military has soldiers zero their rifle at a close target (25 meters).  Thus, the rifle hits dead on at 25 meters, gradually higher and higher at longer distances, then at some distance beyond 200 meters the bullet begins to fall back towards earth and hit dead on at something like 250 meters and lower at distances beyond that.  Generally though, aiming center of mass at a human target and practical distances out to 300 yards, the bullet would hit a little high or low but only by a matter of a few inches effectively killing or wounding an enemy soldier.  This range is known as "battle sight".  For practical purposes, it is a simple matter to zero the rifle at the range at which you shoot, say 100 or 200 yards and obtain a very adaquate "battle-sight".  The exact military sighting method could be described, but it is better that you really understand the concepts to adapt the weapon best to your needs.

To Zero A Rifle 

• Fire 3 shots at a target at a safe shooting range.  If it would be possible to shoot over a back-stop,  zero at a closer distance first. 

• Adjust your front and rear sights accordingly.

Front & Rear Sight Adjustment Relationships

Front Sight - Has Inverse Relationship
   Move sight up to shoot lower, move sight down to shoot higher.

Rear Sight - Has Direct Relationship
   Move sight up to shoot higher, move sight down to shoot lower
   Move sight to the right to move point of impact to the right.

• Adjust your sights the proper number of clicks.  Sights are described in "m.o.a." minutes-of-angle.  That is, a rear sight that is rated 1 m.o.a. will cause bullet impact to move 1" at 100 yards for every click of the sight.  If you are shooting 3 inches to left at 100 yards, move your rear sight 3 clicks right. If shooting at half that distance (50 yards), a 1 m.o.a. click will move it half an inch, and at 200 meters that 1 m.o.a. click will move impact 2 inches.  Minutes of angle differ from gun to gun.   Standard military rifles are rated at 1 m.o.a. while target rifles may be rated at .5 or .25 m.o.a.  So much for the metric system.

• Repeat until you get a good zero.     

More Information

External Ballistics is the science that makes shooting the great fun that it is.  Many great books and other information are available.   Cartridge companies produce many informative booklets as do the manufacturers of reloading equipment.  The NRA fact-book is probably one of the best books on the subjects. 
http://www.nra.org
http://store.nrahq.org/nra/product.asp?dept%5Fid=205&pf%5Fid=PB+01560