There are a large number of types of sights in use on firearms ranging from muskets to rocket launchers. In this section we will limit our universe to modern pistols, revolvers, combat rifles and hunting rifles. Within these classes of firearms we will discuss the following:
Composed of a post front sight, and a notched rear sight. The
shooter must place the firearm at eye level, and aligh the post with
the notch so that it is centered, and the post is flush with the top
of the notch. This is accomplished by looking at the front sight (to
gain the most defined view possible) then adjusting the pistol to
place equal amounts of light on either side of the post while keeping
the post top level with the top of the notch.
There are many variations of this general design. All are to be aimed the same way. Some are designed for low to non-existent light and contain various dots that are to be aligned vertically and/or horizontally. The correct zero for pistol sights is the bullet impacting at the top center of the post at 25 yards. The correct zero for rifle iron sights is 50 yards for rimfire and 100 yards for centerfire with the same relationship between sights and point of impact. Target shooters may change the zero to allow placing the post (front sight) at 6 O'Clock on a round black target to cause impact at the center of the round target.
Adjustment of iron sights is typically either only the rear sight or combination of front and rear. The line of sight from shooter to target is a fixed straight line. The sights are aligned to this line. The barrel of the firearm is adjusted by the sights to deliver the fired bullet to the desired impact location. The rule of thumb is that the rear sight is moved in the direction you want the impact to move, the front sight is moved in the opposite direction to achieve the same movement. Thus on a typical iron sight equipped AR15 the front sight is lowered to raise the point of impact, while the rear sight is moved right to move the point of impact to the right.
Electro Optical Sights
Composed of light pipes, lasers, semi-reflective surfaces and
similar techniques to project to the shooter's eye a reticle in the
image of a dot or other shape (circle/dot, cross hair, triangle,
etc.) The reticle should be placed so it intersects the point of
impact. Rimfire pistols zero at 25 yards, centerfire pistols zero at
50 yards, combat rifle zero at 25 or 50 or 100 yards depending on
mission objectives. First experience with a sight of this type on a
pistol or revolver often results in difficulty picking up and main
taining the sight alignment because they are very sensitive to the
lateral stability of the firearm. Very small changes in how the
firearm is held result in massive changes in where the sight is
pointed. Once the projected image leaves the reflective surface there
is no indication where it might be. Larger surfaces tend to mitigate
the problem, smaller surfaces tend to work better for pistols carried
in holsters. Rifles tend to be much easier to aim and shoot with this
type of sight because the stock positions the shooters head within
the display area and keeps it there. It is only necessary to put the
reticle on the target and execute a good shot to hit the target, the
reticle may be in any place where it can be seen by the shooter.
Another class of electro optical sights are lasers. These sights project a light beam toward the target. They may be designed to project visible light or infra-red (IR) light. The latter for use with night vision equipment. Lasers work best in low to no light, whereas the previous class of sights work well in both darkness and daylight. To use a laser, look at the target, point the laser at the desired point of impact and execute an accurarte shot. To use a reflective type electro optical sight, look at the target, put the reticle on the desired point of impact and execute an accurate shot.
Electro optical sight adjustments are usually marked on the controls and typically turning clockwise on the windage moves the point of impact to the right, clockwise on the elevation control, moves the impact up.
Composed of magnifying lenses, and internal reticles these sights
are made for both pistols and rifles. In pistols they have 'extended
eye relief' to permit the sight being mounted at arms lentgh from the
eye. In rifles, there are two styles, the conventional and a 'Scout'
or forward mounted scope (typically very similar to the pistol
scope). The conventional scope is mounted so that the 'eye relief'
(distance from the ocular lens to the shooter's eye) is typically
about an inch. There usually is a focus ability for the ocular lense
(closest to the shooter's eye) which is used to sharpen the focus of
the reticle. If the magnification is sufficient, there will either a
parallax control, or an objective lense focus. In the former, a knob
on the left is rotated to bring the target image into sharp focus
with the shooter's eye (the reticle should have already been focused
using the ocular focus). In the older objective focus style, the
objective (closest to the object) housing rotates to bring the image
into focus at the reticle plane. The effect is similar, but the means
of achieving the target focus is different between the two. Many
modern rifle scopes are of variable power (magnification). Typically
these scopes are zeroed at 100 yards. Tactical scopes have reticle
markings useful for measuring angles, in either MOA (Minutes of
Angle) or MILS (Milliradians). These markings are used to measure
know size targets to determine the range to the target, as well as
reference points for holding the sight off of the classic point of
aim at the point of desired impact to account for wind, drag, range
Typically telescopic sights are zeroed to place bullet impact at the intersection of the major cross hairs. Tactical and target scopes are typically zeroed at 100 yards and adjusted via their elevation and windage controls to compensate for wind and drag.