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Iron sights are a system of physical alignment markers used as a sighting device to assist the accurate aiming of ranged weapons such as , , , 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 optics manipulation or active illumination, such as , reflector sight, holographic sights, and laser sights. merriam-webster.com; Iron sight; "a metallic sight for a gun as distinguished from a sight depending on an optical or computing system"
Iron sights are typically composed of two components mounted perpendicularly above the weapon's bore axis: a 'rear sight' nearer (or 'proximal') to the shooter's eye, and a 'front sight' farther forward (or 'distal') near the muzzle. During aiming, the shooter aligns their sightline 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 , feature open sights. By contrast, many military 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.Hawks, Chuck. chuckhawks.com; "Choosing the Right Sight". Retrieved July 24, 2008. On many firearms it is the rear sight that is adjustable.
For precision shooting applications such as varminting or sniper, the iron sights are usually replaced by a telescopic sight. Iron sights may still be fitted alongside other sighting devices (or in the case of some models of optics, incorporated integrally) for back-up usage, if the primary sights are damaged or lost.
"Sighting in" is a process in which the sight axis is adjusted to intersect the trajectory of the bullet at a designated distance (typically at 100 yards/), in order to produce a pre-determined point of impact (POI) 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 (though with rather limited precision) for increasing bullet drops at extended distances. Because the sight axis (which is a straight line) and the projectile trajectory (which is a Parabola curve) 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 (a situation known as canting) when aiming or sighting-in.
Rear sights on (such as and ) are usually mounted on a Dovetail joint on the back part of the Gun 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, , 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.
In the case of , the rear sight will be mounted on the frame (for , , and ) or on the Pistol slide (for semi-automatic pistols). 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.
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, with a 10 meter air rifle shooter 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 (a point of impact miss). At , that same misalignment would be magnified 100 times, giving an error of over , 1,500 times the sight misalignment.Calculations assume a sight radius or sighting line 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.
Sights for shotguns used for shooting small, moving targets (such as skeet shooting, trap shooting, and clay pigeon 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 the bead is made of a polished metal such as brass and silver, or a plastic fluorescent material, such as green and orange) round bead is placed at the end of the Gun 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 (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 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.
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. Buckhorn sights 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 Firearms lever-action rifles. 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 (e.g. the shooter is being charged by dangerous big-game), 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.
In the tactical environment, where targets aren't moving across the visual field as quickly, sights do have a role. For many, a fiberoptic front sight is the preferred sighting reference in conjunction with a rear leaf. In this instance, the shotgun is used more like a rifle, allowing intentionally aimed shots. Some even equip their shotguns with open or aperture sights akin to a rifle.
Many shotgun bead sights are designed for a "lemniscate" configuration, where a proper sight picture uses a bead mounted at the midpoint of the barrel in conjunction with a front bead mounted toward the muzzle. Many shotgun manufacturers, such as Browning, calibrate these sighting systems to produce a shotgun pattern that is "dead-on" when the front bead is stacked just above the mid-bead, producing the figure-8 sight picture.
The theory of operation behind the aperture sight is often stated that the human eye will automatically center the front sight when looking through the rear aperture, thus ensuring accuracy. However, aperture sights are accurate even if the front sight is not centered in the rear aperture due to a phenomenon called parallax suppression. This is because, when the aperture is smaller than the eye's pupil diameter, the aperture itself becomes the entrance pupil for the entire optical system of target, front sight post, rear aperture, and eye. As long as the aperture's diameter is completely contained within the eye's pupil diameter, the exact visual location of the front sight post within the rear aperture ring does not affect the accuracy, and accuracy only starts to degrade slightly due to parallax shift as the aperture's diameter begins to encroach on the outside of the eye's pupil diameter. An additional benefit to aperture sights is that smaller apertures provide greater depth of field, making the target less blurry when focusing on the front sight.
In low light conditions the parallax suppression phenomenon is markedly better. The depth of field looking through the sight remains the same as in bright conditions. This is in contrast to open sights, where the eye's pupil will become wider in low light conditions, meaning a larger aperture and a blurrier target. The downside to this is that the image through an aperture sight is darker than with an open sight.
These sights are used on target of several disciplines and on several military rifles such as the Pattern 1914 Enfield and M1917 Enfield, M1 Garand, the No. 4 series Lee–Enfields, M14 rifle, Stgw 57, G3 and the M16 series of weapons along with several others. Rifle aperture sights for military combat or hunting arms are not designed for maximal attainable precision like target aperture sights, as these must be usable under suboptimal field conditions.
The complementing front sight element may be a simple bead or post, but is more often a "globe sight"-type sight, which consists of a cylinder with a threaded cap, which allows differently shaped removable front sight elements to be used. Most common are posts of varying widths and heights or rings of varying diameter—these can be chosen by the shooter for the best fit to the target being used. Tinted transparent plastic insert elements may also be used, with a hole in the middle; these work the same way as an opaque ring, but provide a less obstructed view of the target. High end target front sight tunnels normally also accept accessories like adjustable aperture and optical systems to ensure optimal sighting conditions for match shooters. Some high end target sight line manufacturers also offer front sights with integrated aperture mechanisms.
The use of round rear and front sighting elements for aiming at round targets, like used in ISSF match shooting, takes advantage of the natural ability of the eye and brain to easily align concentric circles. Even for the maximum precision, there should still be a significant area of white visible around the bullseye and between the front and rear sight ring (if a front ring is being used). Since the best key to determining center is the amount of light passing through the apertures, a narrow, dim ring of light can actually be more difficult to work with than a larger, brighter ring. The precise sizes are quite subjective, and depend on both shooter preference and ambient lighting, which is why target rifles come with easily replaceable front sight inserts, and adjustable aperture mechanisms.
Rifles from the late 19th century often featured one of two types of aperture sight called a "tang sight" or a "ladder sight". Since the black powder used in and early cartridges was not capable of propelling a bullet at high speed, these sights had very large ranges of vertical adjustments, often on the order of several degrees, allowing very long shots to be made accurately. The .45-70 cartridge, for example, was tested by the military for accuracy at ranges of up to , which required 3 degrees of elevation. Both ladder and tang sights folded down when not in use to reduce the chance of damage to the sights. Ladder sights were mounted on the barrel, and could be used as sights in both the folded and unfolded states. Tang sights were mounted behind the action of the rifle, and provided a very long sight radius, and had to be unfolded for use, though rifles with tang sights often had open sights as well for close range use. Tang sights often had , allowing adjustment down to a single minute of arc over the full range of the sight.
The downside to adjustable sights is the inherent fragility of the moving parts. A fixed sight is a solid piece of metal, usually steel, and if firmly attached to the gun, little is going to be able to damage it beyond usefulness. Adjustable sights, on the other hand, are bulkier, and have parts that must move relative to the gun. Solid impact on an adjustable sight will usually knock it out of adjustment, if not knock it right off the gun. Because of this, guns for Self-defense or military use either have fixed sights, or sights with "wings" on the sides for protection (such as those on the M4 carbine).
Iron sights used for hunting guns tend to be a compromise. They will be adjustable, but only with tools—generally either a small screwdriver or an Allen wrench. They will be compact and heavily built, and designed to lock securely into position. Target sights, on the other hand, are much bulkier and easier to adjust. They generally have large knobs to control horizontal and vertical movement without tools, and often they are designed to be quickly and easily detachable from the gun so they can be stored separately in their own protective case.
The most common is a rear sight that adjusts in both directions, though military rifles often have a tangent sight in the rear, which a slider on the rear sight has pre-calibrated elevation adjustments for different ranges. With tangent sights, the rear sight is often used to adjust the elevation, and the front the windage. The M16A2 later M16 series rifles have a dial adjustable range calibrated rear sight, and use an elevation adjustable front sight to "zero" the rifle at a given range. The rear sight is used for windage adjustment and to change the zero range.
The most common solution to the problem of glare is a matte finish on the sights. Serrating or bead blasting the sight is a common solution for brightly finished sights, such as blued steel or stainless steel. Matte finishes such as parkerizing or matte black paint can also help. "Smoking" a sight by holding a match or cigarette lighter under the sight to deposit a fine layer of soot is a technique used by many shooters, and special soot-producing lighters are sold for use by competition shooters. Even a thin layer of mud or dirt applied to the sight will help kill the glare, as long as the coating is thin and consistent enough not to change the shape of the sights.
Many target sights are designed with vertical or even undercut front sight blades, which reduces the angles at which light will produce glare off the sight—the downside of these sights is that they tend to snag on clothing, branches, and other materials, so they are common only on target guns. Sight hoods reduce the chances of snagging an undercut sight and are common on some types of rifles, particularly lever-action rifles, but they are prohibited in some shooting disciplines.
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