Iron sight



Iron sights are a system of shaped alignment markers (usually metal) used as a sighting device to assist in the aiming of a device such as a firearm, crossbow, or telescope, and exclude the use of optics as in telescopic sights or reflector (reflex) sights. Iron sights are typically composed of two component sights, formed by metal blades: a rear sight mounted perpendicular to the line of sight and consisting of some form of notch (open sight) or aperture (closed sight); and a front sight that is a post, bead, or ring. Civilian defensive, hunting, and police firearms usually feature open sights, while many military battle rifles employ aperture sights. On many firearms the rear sight is adjustable for elevation or windage.

The earliest and simplest iron sights are fixed and cannot be easily adjusted. Many iron sights are designed to be adjustable, so that the sights can be adjusted for windage and elevation. For precision applications such as hunting or sniping the iron sights are usually replaced by a telescopic sight. Iron sights may still be fitted alongside a telescopic sight and are referred to as Back Up Iron Sights (Acronym B.U.I.S).

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.

Types of sights
Iron sights can be broken into two basic categories that include most types: Open sights which use a notch of some sort as the rear sight, and aperture sights use some form of a circular hole. Sights of both types can also be classified as fixed sights or adjustable sights.

Fixed and adjustable sights
Fixed sights are sights that are not adjustable. For instance, on many revolvers, the rear sight consists of a fixed sight that is a groove milled into the top of the gun's receiver. Adjustable sights are designed to be adjustable for different ranges, for the effect of wind, or to compensate for varying cartridge bullet weights or propellant loadings, which alter the round's velocity and external ballistics and thus its trajectory and point of impact. Sight adjustments are orthogonal, so the windage can be adjusted without impacting the elevation, and vice versa. If the firearm is held canted instead of level when fired, the adjustments are no longer orthogonal, so it is essential to keep the firearm level for best accuracy.

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.

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. The post or bead is placed in the rear sight notch, and the target is placed above and centered on the aligned sights. 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. 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 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. 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.

Advantages and disadvantages
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, partridge, '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.

Aperture sights


Aperture sights, also known as peep sights, range from the ghost ring sight, whose thin ring blurs to near invisibility (hence ghost), to target aperture sights that use large disks or other occluders with pinhole-sized apertures. In general, the thicker the ring, the more precise the sight, and the thinner the ring, the faster the sight. The image to the right shows a shooter's eye view of the sight picture taken through large and small diameter apertures. The large diameter aperture provides a much brighter image of the target, and the ghosting of the rear ring is evident. The smaller aperture, while providing a much darker image of the target, provides a much greater depth of field (see pinhole camera for an explanation of this effect), yielding a much sharper image of the target. The theory of operation behind the aperture sight is that the human eye will automatically center the front sight when looking through the rear aperture, thus ensuring accuracy.

These sights are used on target rifles of several disciplines and on several military rifles such as the M1 Garand, the No. 4 series Enfields and the M16 series of weapons along with several others. Peep sights are very effective in poor lighting, and have found favor with a few hunters who hunt in heavy cover.

Ghost rings
The ghost ring sight is considered by some to be the fastest type of aperture sight. It is fairly accurate, easy to use, and obscures the target less than nearly all other non-optical sights. Because of this, ghost ring sights are commonly installed on combat shotguns and submachine guns (and to a much lesser extent, rifles). The ghost ring is a fairly recent innovation, and differs from traditional aperture sights in the extreme thinness of the rear ring, and the slightly thicker front sight. The thin ring minimizes the occlusion of the target, while the thicker front post makes it easy to find quickly.

A ghost ring can also take form of a tube ranging from 3 to 6 cm long, with the post mounted at the inside end of the tube. This type of ghost ring is used when there can be no front sight at the front end of the barrel. It is slightly slower to use, because the shooter's eye has to focus close on the post, then focus on the target. However, in situations when extremely fast sighting is required, the shooter can simply look through the tube, and it is simple to see if the view is straight down the tube.

Target aperture sights


Target aperture sights are designed for maximum precision. The rear sight element (often called diopter) is usually a large disk (up to 1 inch or 2.5 cm in diameter) with a small hole in the middle, and is placed close to the shooter's eye. High end target diopters normally accept accessories like adjustable diopter aperture and optical filter systems to ensure optimal sighting conditions for match shooters. Typical modern target shooting diopters offer windage and elevation corrections in 2 mm to 4 mm increments at 100 m. Some ISSF (Olympic) shooting events require this precision level for sighting lines, since the score of the top competitors last 10 shots is expressed in tenths of scoring ring points.

The complementing front sight element may be a simple bead or post, but is more often a globe 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 muzzleloaders and early cartridges was not capable of propelling a bullet at high velocity, 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 1500 yards, which required 3$1⁄3$ 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 vernier scales, allowing adjustment down to a single minute of arc over the full range of the sight.

Aperture sights on military rifles use a larger aperture with a thinner ring, and generally a simple post front sight. The extreme case of this is the ghost ring sight, a relatively recent innovation that may be the fastest type of iron sight to use, while still providing a degree of precision comparable to or better than most open sights. Ghost ring sights are commonly found on riot and combat shotguns and customized handguns, and they are also gaining ground as a backup sighting system on rifles.

Shotgun beads
Since shotgun beads are only used by the peripheral vision, generally a larger, brighter bead works best. Fiber optic sights are becoming popular for shotguns, as they greatly increase the brightness of the bead by collecting light and directing it to the shooter's eye. Since the "rear sight" in the case of a shotgun is the shooter's eye position, adjusting the "sights" on a shotgun consists primarily of adjusting the stock to fit the shooter as well as possible.

Bead sights are inferior in practical accuracy compared to rifle and ghost-ring sights, as they provide no rear sight to verify correct alignment with the front sight. While bead sights may be acceptable on sporting shotguns, they are best avoided for use on combat shotguns.

The primary advantage of bead sights is that they do not interfere with the shooter's field of vision, allowing extremely rapid target acquisition, which is essential for fast moving targets such as clays and game. They are less expensive than rifle and ghost-ring sights.

Enhancements
While iron sights are basically very simple, that simplicity also leads to a staggering variety of different implementations. In addition to the purely geometric considerations of the front blade and rear notch, there are some factors that need to be considered when choosing a set of iron sights for a particular purpose.

Glare reduction
Glare, particularly from the front sight, can be a significant problem with iron sights. The glare from the front sight can increase the apparent brightness of the light bar on one side of the sight, causing windage errors in aiming, or lower the apparent height of the front sight, causing elevation errors in aiming. Since the direction of the ambient light is rarely constant for a shooter, the resulting changing glare can significantly affect the point of aim.

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 common technique used by many shooters, and in fact special soot producing cigarette type 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 only common 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 such as some classes of handgun metallic silhouette shooting.

Contrast enhancements


While target shooters generally prefer a matte black finish to their sights, to reduce the chance of glare and increase the contrast between the sights and the light bars, black sights don't offer good visibility with dark targets or in low light conditions, such as those often encountered in hunting, military, or self-defense situations. A variety of different contrast enhancements to the basic Patridge type sight and others have been developed to address this deficiency. It should be noted that the contrast enhancement of the front sight has to be somewhat larger compared to the contrast enhancement(s) used for the rear sight if all contrast enhancements should appear about equally large from the shooters perspective.


 * Three-dot: On semi-automatic handguns, the most common type of enhancement is a bright white dot painted on the front sight near the top of the blade, and a dot on each side of the rear sight notch. In low lighting conditions the front sight dot is centered horizontally between the rear sight dots, with the target placed above the middle (front) dot. Some sight vendors offer differently colored dots for the front and rear sights.
 * White outline rear: A contrast variation which uses a dot front sight with a thick and bright white outline around the rear sight notch.
 * Straight Eight: Heinie Specialty Products produces a variant of high visibility sights in which a single dot front sight and a rear notch with a dot below can be lined up vertically to form a figure "eight".
 * Sight inserts: Popular on revolvers, this enhancement consists of a colored plastic insert in the front sight blade, usually red or orange in color.
 * Bar/Dot or Express sight: Similar to the Straight Eight type, this type of sight is traditional on express rifles and is also found on some handguns. The open, V-shaped rear allows for faster acquisition and wider field of view, though less accurate for longer range precision type shooting. The dot on the front sight is aligned or set directly above the vertical bar on the rear sight, commonly referred to as "dotting the 'I'".
 * Gold bead: Preferred by many competitors in IPSC and IDPA shooting.
 * Night sights: On tactical firearms, the contrast enhancements can consist of trasers containing tritium gas, which emit a dim light due to the radioactive decay of the tritium, and fluorescent material. Self-luminous tritium sights provide vital visibility in extremely low light situations where normal sights would be degraded or even useless. The tritium glow is not noticeable in bright conditions such as during daylight however. As a result, some manufacturers have started to integrate fiber optic sights with tritium vials to provide bright, high-contrast firearms sights in both bright and dim conditions.
 * Fiber optic: A growing trend, started on air rifles and muzzleloaders, is the use of short pieces of optical fiber for the dots, made in such a way that ambient light falling on the length of the fiber is concentrated at the tip, making the dots slightly brighter than the surroundings.This method is most commonly used in front sights, but many makers offer sights that use fiber optics on front and rear sights. Fiber optic sights can now be found on handguns, rifles, and shotguns, both as aftermarket accessories and a growing number of factory guns.

Concealed carry
Specific to handguns is the issue of concealed carry. Police and military personnel generally carry their handguns in open holsters that provide easy and unobstructed access. Where concealed carry by citizens is allowed, those citizens often choose or are required to carry their firearms hidden, under clothing or some other form of cover (such as an untucked shirt, fanny pack or purse, for example). The high contrast, sharp edges popular with target shooters can be dangerous on a concealed firearm—-not only can they tear skin and clothing when drawn, but if they become tangled during the draw, entanglement can have lethal consequences for the shooter.

The challenge a sight designer faces for a concealable handgun is to pick the right compromise between high visibility and minimum hindrance. Rather than the vertical or even undercut front sights found on target handguns, the front sight of a concealed carry gun will be ramped, or sloped gradually upwards from the rear to the front. The rear sight will not be a vertical plate, but will have depth to also provide a gentle slope, and the sight will fit the gun more closely, with no gaps or overlaps to snag, or to collect dirt and sweat which can cause corrosion.

Design criteria for concealed handgun sights also consider the challenging conditions under which the sights may potentially be used—-low light, time pressure, close range, and with the operator under significant stress due to imminent threat. Some unusual solutions have been devised, such as the trapezoidal sights used by Steyr M (shown as G in Types of Iron Sights), or the "gutter sight" used by the highly modified ASP S&W model 39. Colt's Manufacturing Company also produces a line of sub-compact pistols with what they call a trench sight; it is simply a channel cut into the length of the top of the slide, for fast but short range target acquisition.

Adjusting for accuracy
If the sights are not aligned correctly, then the sights should be adjusted to bring the line of sight to meet the point of impact. Theoretically, this can be done with a single shot—clamp the firearm into a vise, fire one shot, then adjust the sights so they are pointing at the hole in the target. In reality, it generally takes a number of shots to establish a group, then the sights are adjusted to move the line of sight closer to the group, and the process is repeated iteratively until the sights are correctly aligned.

The general rule is the rear sight is moved in the SAME direction you wish to move the point of impact. For example, if the point of impact is LEFT and BELOW the target, and you wish to move the point of impact to the center, then move the rear sight RIGHT and UP. The front sight moves the opposite direction, so it would move LEFT and DOWN.

Detailed instructions for adjusting the sights:
 * To move the line of sight DOWN (the shot hit BELOW the point of aim) the REAR sight is RAISED or the FRONT sight is LOWERED.
 * To move the line of sight UP (the shot hit ABOVE the point of aim) the REAR sight is LOWERED or the FRONT sight is RAISED. (Example below)
 * To move the line of sight LEFT (the shot hit LEFT of the point of aim) the REAR sight is moved RIGHT, or the FRONT sight is moved LEFT.
 * To move the line of sight RIGHT (the shot hit RIGHT of the point of aim) the REAR sight is moved LEFT, or the FRONT sight is moved RIGHT.

Many target sights have click adjustments, where a detent in the adjustment screws allows the sight to move the line of sight a certain angular distance with each click. This distance is usually specified in minutes of arc, which translate to approximately 1 inch at 100 yards. On a firearm with 1 minute clicks, then, it would take 1 click to move 1 inch at 100 yards, 2 clicks to move 1 inch at 50 yards, 4 clicks to move 1 inch at 25 yards. If click adjustments are not available, or the click interval is not known, then the distance to lengthen or shorten the sight for a given point of aim adjustment is:

D1 / R1 = D2 / R2

For rear sight adjustments:
 * D1 is the distance between point of aim and point of impact.
 * R1 is range from front sight to target.
 * D2 is the length the rear sight must change by.
 * R2 is the distance between front and rear sights.

For front sight adjustments:
 * D1 is the distance between point of aim and point of impact.
 * R1 is range from rear sight to target.
 * D2 is the length the front sight must change by.
 * R2 is the sight radius distance between front and rear sights.

This formula calculates the MAGNITUDE ONLY of the sight height change; refer to the instructions above to find the correct direction for the adjustment (front or rear sight, longer or shorter). Likewise, all distances must be in the same units. That is, if a change in inches to the sight height is desired, and one is shooting on a 100-yard range, then R1 (100 yd) must be converted to inches (100 × 36 = 3600 inches) before using this distance in the equation.

"An example: Consider a rifle with a distance between front and rear sights of 26.25 inches, firing on a 50-yard (1800 in) range, with point of impact 5.3 inches too high on the target, having a front sight blade that is 0.505 inches high mounted in a dovetail. How much must the front sight blade height be changed by to fix this problem? (It will be assumed that the muzzle of the rifle intrudes into the range space for following typical gun range safety protocols, and the rear sight is hence 50 yards from the target.)"

D2 = R2(D1/R1) = 26.25(5.3/1800) = 0.077" (magnitude of change to front sight height)

"Since the gun is hitting too high, the front sight must be lengthened by this much per the instructions cited previously; hence, the front sight must be replaced with a blade that is 0.505' + 0.077' = 0.582' high. With this correction, the rifle will hit the desired point of impact, all other factors being equal."