Iron sights
Iron sights are a system of physical alignment markers used as a sighting device to assist the accurate aiming of ranged weapons such as firearms, airguns, crossbows, and bows, or less commonly as a primitive finder sight for optical telescopes. Iron sights, which are typically made of metal, are the earliest and simplest type of sighting device. Since iron sights neither magnify nor illuminate the target, they rely completely on the viewer's naked eye and the available light by which the target is visible. In this respect, iron sights are distinctly different from optical sight designs that employ optical manipulation or active illumination, such as telescopic sights, reflector sights, holographic sights, and laser sights.
Iron sights are typically composed of two components mounted perpendicularly above the weapon's bore axis: a 'rear sight' nearer to the shooter's eye, and a 'front sight' farther forward near the muzzle. During aiming, the shooter aligns their line of sight past a gap at the center of the rear sight and towards the top edge of the front sight. When the shooter's line of sight, the iron sights, and target are all aligned, a 'line of aim' that points straight at the target has been created.
Front sights vary in design but are often a small post, bead, ramp, or ring. There are two main types of rear iron sight: 'open sights', which use an unenclosed notch, and 'aperture sights', which use a circular hole. Nearly all handguns, as well as most civilian, hunting, and police long guns, feature open sights. By contrast, many military service rifles employ aperture sights.
The earliest and simplest iron sights were fixed and could not be easily adjusted. Many modern iron sights are designed to be adjustable for sighting in firearms by adjusting the sights for elevation or windage. On many firearms it is the rear sight that is adjustable.
For precision shooting applications such as varminting or sniping, the iron sights are usually replaced by a telescopic sight. Iron sights may still be fitted alongside other sighting devices for back-up usage, if the primary sights are damaged or lost.
Principles
In the case of firearms, where the projectile follows a curved ballistic trajectory below the bore axis, the only way to ensure it will hit an intended target is by aiming at the precise point on the trajectory at that target's intended distance. To do that, the shooter aligns their line of sight with the front and rear sights, forming a consistent 'line of aim' and in turn producing what is known as the 'point of aim' within their own field of view, which then gets pointed directly at the target. The physical distance between the front and rear sights is known as the 'sight radius', the longer of which produces smaller angular errors when aiming."Sighting in" is a process in which the sight axis is adjusted to intersect the trajectory of the bullet at a designated distance, in order to produce a pre-determined point of impact at that distance, known as a "zero". Using that "zero" as a default reference, the point of aim can be readily re-calibrated to superimpose with the bullet's point of impact when shooting at different distances. Modern iron sights can all provide some horizontal and vertical adjustments for sighting-in, and often have elevation markings that allow the shooter to quickly compensate for increasing bullet drops at extended distances. Because the sight axis and the projectile trajectory must be within the same vertical plane to have any chance of intersecting, it will be very difficult to shoot accurately if the sights are not perpendicularly above the gun barrel when aiming or sighting-in.
Long gun sights
Rear sights on long guns are usually mounted on a dovetail slot on the back part of the barrel or the receiver, closer to the eye of the shooter, allowing for easy visual pick-up of the notch. Front sights are mounted to the front end of the barrel by dovetailing, soldering, screwing, or staking very close to the muzzle, frequently on a "ramp". Some front sight assemblies include a detachable hood intended to reduce glare, and if the hood is circular, then this provides a reference where the eye will naturally align one within the other.Handgun sights
In the case of handguns, the rear sight will be mounted on the frame or on the slide. Exceptions are possible depending on the type of handgun, e.g. the rear sight on a snub-nose revolver is typically a trench milled into the top strap of the frame, and the front sight is the to-be-expected blade. Certain handguns may have the rear sight mounted on a hoop-like bracket that straddles the slide.With typical blade- or post-type iron sights, the shooter would center the front sight's 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 focal plane at a time, 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. The general advice, however, is to focus on the front sight.
Shotgun sights
Sights for shotguns used for shooting small, moving targets 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 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 with the focus always on the target, and the unfocused image of the barrel and bead are placed below the target 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 shots can allow an effective hit even if there is some aiming error. Some shotguns also provide a mid-bead, which is a smaller bead located halfway down the rib, which allows more feedback on barrel alignment. Some shotguns may also come equipped with rifle-style sights. These types of sights are typically found on shotguns intended for turkey hunting.Parallax and precision
Due to parallax, 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, a 10 meter air rifle shooter is trying to hit the 10 ring, which is merely a diameter dot on the target at and with a diameter pellet, an error of only in sight alignment can mean a complete miss. At, that same misalignment would be magnified 100 times, giving an error of over, 1,500 times the sight misalignment. Increasing the sight radius helps to reduce eventual angular 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. With the front sight on the front end of the barrel, sight radius may be increased by moving the rear sight from the barrel onto the receiver or tang.Types
Open sights
Open sights generally are used where the rear sight is at significant distance from the shooter's eye. They provide minimum occlusion of the shooter's view, but at the expense of precision. Open sights generally use either a square post or a bead on a post for a front sight. To use the sight, the post or bead is positioned both vertically and horizontally in the center of the rear sight notch. For a center hold, the front sight is positioned on the center of the target, bisecting the target vertically and horizontally. For a 6 o'clock hold, the front sight is positioned just below the target and centered horizontally. A 6 o'clock hold is only good for a known target size at a known distance and will not hold zero without user adjustment if these factors are varied. From the shooter's point of view, there should be a noticeable space between each side of the front sight and the edges of the notch; the spaces are called light bars, and the brightness of the light bars provides the shooter feedback as to the alignment of the post in the notch. Vertical alignment is done by lining up the top of the front post with the top of the rear sight, or by placing the bead just above the bottom of the V or U-notch. If the post is not centered in the V or U notch, the shot will not be accurate. If the post extends over the V or U-notch it will result in a high shot. If the post does not reach the top of the V or U-notch it will result in a low shot.Patridge sights, named after inventor E. E. Patridge, a 19th-century American sportsman, consist of a square or rectangular post and a flat-bottomed square notch and are the most common form of open sights, being preferred for target shooting, as the majority of shooters find the vertical alignment is more precise than other open sights. V-notch and U-notch sights are a variant of the patridge which substitute a V- or U-shaped rear notch. Other common open sight types include the buckhorn, semi-buckhorn, and express.
s have extensions protruding from either side of the rear sight forming a large ring which almost meets directly above the "V" of the notch. The semi-buckhorn is similar but has a wider gently curving notch with the more precise "V" at its center and is standard on classic Winchester and Marlin lever-action rifles. They allow the 'Kentucky windage' technique to provide rapid sighting for two or three ranges, without stopping for adjustment. When the fore sight is aligned with the horns of the rear sight, the rifle is sighted for long range. When aligned with the base of the horns, the range is near. It can also give an intermediate range picture.
Express sights are most often used on heavy caliber rifles intended for the hunting of dangerous big game, and are in the form of a wide and large "V" with a heavy white contrast line marking its bottom and a big white or gold bead front sight. These sights do not occlude the target as much as some other styles which is useful in the case of a charging animal. In cases where the range is close and speed far outweighs accuracy, the front sight is used like a shotgun bead; the rear sight is ignored, and the bead is placed on the target. When more time is available, the bead is placed in the "V" of the rear sight.
Open sights have many advantages: they are very common, inexpensive to produce, uncomplicated to use, sturdy, lightweight, resistant to severe environmental conditions, and they do not require batteries. On the other hand, they are not as precise as other forms of sights, and are difficult or impossible to adjust. Open sights also take much more time to use—the buckhorn type is the slowest, patridge, "U" and "V" type notch sights are only a bit quicker; only the express sight is relatively fast. In addition, open sights tend to block out the lower portion of the shooter's field of view by nature, and because of the depth of field limitations of the human eye, do not work as well for shooters with less than perfect vision.