Theory
In the case of firearms, where the bullet follows a Newtonian trajectory, front and rear sights must be aligned with the line of sight of the shooter, calibrated to the distance of the target and the trajectory of the bullet, so that the bullet hits the target. Iron sights provide horizontal and vertical reference points that allow the shooter to train the weapon.
Rear sights are usually mounted in a dovetail on the barrel or receiver, closer to the eye of the shooter, allowing for easy visual pick-up of the notch. Front sights are mounted to the barrel by dovetailing, sweat soldering, screwing, or staking close to the muzzle, frequently on a ramp. Some front sight assemblies include a detachable hood intended to reduce glare.
With typical blade iron sights, the shooter should center the front post in the notch of the rear sight and the tops of both sights should be level. Since the eye is only capable of focusing on one plane, and the rear sight, front sight, and target are all in separate planes, only one of those three planes can be in focus. Which plane is in focus depends on the type of sight, and one of the challenges to a shooter is to keep the focus on the correct plane to allow for best sight alignment.
Even a tiny error in the angle of sight alignment results in a trajectory that diverges from the target on a trajectory directly relative to the distance from the target, causing the bullet to miss the target; for example with an Olympic 10 metre air rifle shooter trying to hit the 10 ring, which is merely a 0.5 mm diameter dot on the target at 10 meters and with a 4.5 mm diameter pellet, an error of only 0.2 mm in sight alignment can mean a complete miss (a 3 mm point of impact miss). At 1,000 meters, that same 0.2 mm misalignment would be magnified 100 times, giving an error of over 300 mm (1500 times the sight misalignment of 0.2 mm). (Calculations assume a 660 mm sight radius or sighting line) A long sighting line helps to reduce eventual angle errors and will, in case the sight has an incremental adjustment mechanism, adjust in smaller increments when compared to a further identical shorter sighting line.
Sights for shotguns used for shooting small, moving targets (wing shooting or clay shooting) work quite differently. The rear sight is completely discarded, and the rear reference point is provided by the correct and consistent positioning of the shooter's head. A brightly colored (generally brass or silver colored, white, or a fluorescent shade) round bead is placed at the end of the barrel. Often this bead will be placed along a raised, flat rib, which is usually ventilated to keep it cool and reduce mirage effects from a hot barrel. Rather than being aimed like a rifle or handgun, the shotgun is pointed—the focus is always on the target, and the unfocused image of the barrel and bead are placed below the target (the amount below depends on whether the target is rising or falling) and slightly ahead of the target if there is lateral movement. This method of aiming is not as precise as that of a front sight/rear sight combination, but it is much faster, and the wide spread of shot allows a hit even if there is some error in aim. Some shotguns also provide a mid-bead, which is a smaller bead located halfway down the rib, which allows more feedback on barrel alignment.
Read more about this topic: Iron Sight
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—Ralph Waldo Emerson (1803–1882)
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