Thought I'd chime in here for a few brief comments.
If the rifle is "zeroed" through the iron sights using a particular
sight picture, then that rifle should shoot to the same POI with
subsequent users if they adopt
the same sight picture that was used to zero the rifle. The
iron sights and barrel are attached to the upper receiver and are
fixed in relation to each other, so it does not matter how the
shooter's head is situated with respect to the rifle, as long the
dominant (aiming) eye is in the correct position to acquire the
same sight picture each and every time.
In practice, however, not everyone might use the same sight
picture. For example, one shooter might prefer to have the entire
bullseye sit atop the front sight post, while another might prefer
that the top of the front sight post horizontally bisect the exact
center of the bullseye. When zeroing a firearm, the shooter's
preferred sight picture has to be accommodated.
Other factors that might work against achieving a repeatable zero
in a particular firearm might include:
Variances in the
ammunition. Manufacturing variances in the mass production
of ammunition (coupled with the occasional lapse in quality
control) might result in inconsistent powder charges, projectile
weights and seating depths. The resulting variations in muzzle
velocity would cause the POI to wander from the POA. Granted, the
5.56mm cartridge is a very flat-shooting round, and differences in
muzzle velocity might not change the POI significantly at 100
yards, but at 200 yards and beyond, the changes in POI do become
noticeable.
Military-issue and military-surplus ammunition aren't exactly known
for holding tight manufacturing tolerances. I've measured variances
of up to 400 fps with milsurp M193 (55 gr.) ammunition (and
slightly less with M855 [62 gr.] rounds) out of the barrel of the
same AR. That's why AR shooters who want to achieve a repeatable
zero to maximize accuracy, handload their own ammunition.
Weather
conditions. Wind, temperature and humidity do, to varying
degrees, affect the ballistics of the projectile. A gusty crosswind
would wreak havoc with your POI, especially at extended shooting
distances. An explanation of how temperature and humidity affect
ballistics can be found here.
Fortunately (or unfortunately, depending on your point of view),
it's always hot and humid in Singapore, so the effects of
temperature and humidity can almost be regarded as a constant.
MG