Sighting in.

What is it about zeroing that so baffles so many shooters?

Time and again I've helped people zero their rifles at the range and had them effusively thank me for performing my magic.

It's not that hard.  In fact, the instructions are marked on the sights for many guns and scopes.

In its most basic instance what we are trying to do with zeroing is to align the bullet's flight path with the line of sight.

This is a little complicated in principle, but pretty simple in application.  Bullets do not travel in a straight line, they travel in an arc.  If the barrel is level the bullet begins to descend as soon as it exits the muzzle.  If the sights are also level, then the round will hit below the point of aim.  To make them match you have to either point the sights down or the barrel up.  This actually amounts to the same thing.

Most people seem to get this part.

On a scope the adjusters are normally labeled telling you what direction makes the impact point go what way and how far per click at what range.  i.e. "1/4" Left at 100 yards".  Nearly every shooter I've seen can handle that no problem.  Most troubles with scopes seem to be getting on the paper in the first place because their scope is not mounted well.

Iron sights are where people have the most trouble heading the right way.  Even if there's an arrow pointing clockwise and an engraved 'R' they don't know what direction to turn the wheel.

Here's the "magic" formula for iron sights!  Move the rear sight in the same direction you want the impact to move.  Move the front sight in the opposite direction you want the impact to move.

Simple.

Example.  An AR-15.  The AR adjusts for elevation at the front sight and windage at the rear.  To make the impact move up I need to screw the front sight down, or clockwise.  To make the impact move right I need to turn the windage drum clockwise; thus moving the rear sight right.

I think what gets some folks confused with the AR is rear sights based on the M16A2.  These sights have an adjustment for range on them.  You don't adjust that to zero the gun.