Victor,
The thing to keep in mind is that your bullet crosses your line of sight twice- once as it rises through the line of sight fairly close to the muzzle, at 25-30 yards or so depending on caliber and zero, and again as it descends through the line of sight in its parabolic plunge back to earth. Usually the second point at which the bullet crosses the line of sight is what most people think of as that rifle's zero.
We're really dealing with three "lines" here- two of them perfectly straight, and the third curved- the third line being the trajectory of the bullet, which is a parabola. The first line (one of the straight ones) is the line of the bore, which extends to infinity from the rifle's muzzle in the line created by the bore itself. The second straight line is the line of sight, which also extends to infinity and is the line created by the rifle's sights.
The two straight lines (line of sight and line of bore) intersect at only one point- they're straight lines extending to infinity, remember? On a conventional rifle, the sights are placed above the bore, and in order to hit anything, the rifle has to be sighted in. That means that the line of sight is depressed to the point where it intersects the line of the bore (or the line of the bore is elevated until it intersects the line of sight-whichever).
That means that when the bullet is launched from the muzzle, it is angled upward by the line of the bore to cross the line of sight. Since bullets travel in parabolas and not straight lines, however, that means it crosses the line of sight TWICE- once as it rises, the second time as it descends.
Maybe reading
http://www.chuckhawks.com/bullet_trajectory.htm will help explain this better.
If you're shooting at tiny targets at exactly 100 yards, a precise 100 yard zero matters. My neighbor's kids used to like to shoot my .223 bolt gun with its fixed 6X scope, and their favorite trick was to shoot (from a bench rest) through the cap of a 2-liter bottle full of water laid on its side at the 100 yard line. For their sakes I kept this particular rifle zeroed at exactly 100 yards. The results of a precise hit weren't much different from a miss of an inch or two, but finding the cap with a .22" hole in it gave them a real charge.
Zero depends mostly on the purpose of the rifle. If you're shooting at deer or other large game at typical hunting ranges, you're handicapping yourself unnecessarily most of the time to zero precisely at one hundred yards. The exception (and there's always at least one exception) is if you're using a scope with a reticle precisely calibrated to your caliber and load, with stadia lines or other markings for various ranges.
In the case of the typical hunting rifle, it's usual to zero about 2.5" high at 100 yards. That maximizes the distance at which your rifle will be "minute of deer" without having to hold over. See
http://www.rockymountainhunting.com/articles/articles-scope-sighting.html for more a more detailed explanation.
And for more, see
http://www.chuckhawks.com/rifle_trajectory_table.htm .
My current project is a new Savage 93R in .17HMR. It's outfitted with a Sightron II 4-16x42 scope. After looking long and hard at the drop tables at
http://www.varmintal.com/17hmr.htm , I decided to zero it at 1.5" high at 100 yards. That puts it dead on at 165 yards, which is a pretty good haul for a pipsqueak rimfire round. But it means the bullet is never more than 1.5" above line of sight out to that magic 165 yards, and it ought to be no more than 1.5" low out as far as almost 200 yards. Since this is a varmint caliber suited for small varmints, that means out to about 200 yards or so I shouldn't have to worry much about holding over. That's not too shabby for a little bitty rifle shooting little bitty (17 grain) bullets. I haven't had access to a 200 yard range to confirm that yet, but I will...
hth,
lpl
