Iron sight
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The term iron sights refers to the open, unmagnified system used to assist the aiming of a variety of devices, usually those intended to launch projectiles, such as firearms, airguns, and crossbows; they are also used on many telescopes to help point the telescope at a desired target. Iron sights usually consist of some form of notch or aperture in the rear sight and a post, bead or ring in the front sight. Often, the rear sight is adjustable for windage and/or elevation, though in many military rifles, the front sight is also adjustable.
This article will concentrate on firearms sights; the principles described are equally applicable to any device which needs aiming. For the sake of brevity, the term gun will be used to indicate any device aimed by iron sights, the term shooter will be used for the operator of said device, and the term target will be the object at which the device is being aimed.
The term is also used in video games, specifically first-person shooters, to describe a system where the player's aim is not assisted by on-screen crosshairs (also known as a reticle) and thus gunfire must be aimed using the weapon models' iron sights.
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[edit] How iron sights work
Iron sights work by providing horizontal and vertical reference points that allow the shooter to align the gun parallel to the shooter's line of sight. Once the sights are aligned with each other, they are placed in correct relation to the target. This places the gun at a precise angle to the line of sight to the target. With appropriate compensation for range to the target, parallax between the iron sights and the gun's bore, and the trajectory of the projectile, a shot fired will hit the target.
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. A tiny error in sight alignment can be multiplied hundreds or thousands of times by the time the projectile reaches the target; for example, with an Olympic-class air rifle shooter trying to hit the 10 ring, which is merely a dot on the card, with a 4.5 mm diameter pellet at 10 meters, an error of 0.2 mm in sight alignment can mean a miss. At 1000 meters, that same 0.2 mm misalignment would be magnified 1500 times, giving an error of over 300 mm. (Calculations assume a 660 mm sight radius)
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.
[edit] Adjusting iron sights
Many iron sights are designed to be adjustable, so that the sights can be tuned to match a particular cartridge and distance. Even non-adjustable sights can often be adjusted, though the services of a gunsmith might be needed. Generally, adjustable sights provide a means to adjust the horizontal or windage and/or the vertical or elevation of one or both sights. The 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 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.
[edit] Types of iron sights
Iron sights are broken into two basic categories that include most types. Open sights use a notch of some sort as the rear sight, while aperture sights use a circular hole. Wing and clay-shooting shotgun sights are called shotgun beads, or simply beads.
[edit] 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 isn't 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 are the most common sights used for target pistol shooting, as most shooters find the vertical alignment is more precise than that of other types of open sight. Express sights are generally considered the fastest of the open sights, as the wide, shallow "V" obscures less of the shooter's vision, and the usually large front bead (similar to a shotgun sight) is easy to find in a hurry. The express sight is the only type of open sight where it is considered acceptable to focus on the target, and not the front sight. In cases where speed far outweighs accuracy (e.g. the shooter is being charged by a Cape Buffalo), then the front sight is used like a shotgun bead; the rear sight is ignored, and the bead is placed below the target. When more time is available, then the focus should be on the bead, which allows more precise placement of the bead in the "V" of the rear sight.
[edit] 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 with pinhole-sized apertures. In general, the thicker the ring, the more precise the sight, and the thinner the ring, the faster the sight.[1]
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.[1]
The theory of operation behind the aperture sight is that the human eye will automatically center the front sight when looked through the rear aperture, thus ensuring accuracy.[1]
[edit] Ghost rings
For many shooters, the ghost ring sight is 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 sub-machine 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 3-6 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 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.
[edit] Target aperture sights
Target aperture sights are designed for maximum precision, and the rear sight is usually a large disk (up to 1 inch or 2.5 cm in diameter) with a small hole in the middle, and placed close to the shooter's eye. The front sight 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 a number of differently shaped front sights 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 inserts 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. 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 rear apertures.
Rifles from the late 1800s 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.
[edit] 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 are less expensive than rifle and ghost-ring sights.
[edit] Iron sight 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.
[edit] 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, which cover the front and/or rear sight with a small tube, serve both to reduce glare and reduce the chances of snagging an undercut sight. Hooded front sights 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.
[edit] Fixed vs. adjustable sights
All sights are adjustable to some extent; most are dovetailed into the gun, and can be "drifted" back and forth with a hammer and punch. Even in the case of sights that are milled directly into the gun, as are often found on compact revolvers, careful work with a file can change the height of the front or rear sight, and the notch in the rear sight can be widened on one side only to move the center to the side.
Since different cartridge loadings (heavier or lighter bullet, higher or lower velocity) will change the trajectory of the bullet, it is common for rifles and many handguns to have adjustable sights. Generally only the rear sight is adjustable, although on graduated sights the rear sight is generally "click"-adjustable, and the front sight adjusts to "zero" the rifle for a given load. Once zeroed, the rear sight can be adjusted in graduated intervals to provide correct point of aim for a variety of distances. These graduated rear sights are generally called "tangent sights" when they are open sights mounted on the barrel, and "tang sights" when they are peep sights mounted behind the receiver.
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. A 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 copy shown above).
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.
[edit] 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 show up well on dark targets or in low light conditions, such as those often encountered in hunting, military, or self defense situations. This has led to a variety of different contrast enhancements to the basic Patridge type sight and others. On revolvers, this has generally taken the form of a colored plastic insert in the front sight blade, usually red or orange in color. On semi-automatic handguns, the most common type 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. These are called three-dot sights, and when the contrast is too low to use like standard Patridge sights, the front sight dot is centered between the rear sight dots, and the target is placed over the middle dot. Many variations on this exist, such as using a white outline of the rear sight notch, or a single dot below the rear notch, which are lined up to form a figure "8". On military and police guns, the dots are often trasers containing tritium gas and fluorescent material, which emits a dim light due to the radioactive decay of the tritium. 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 fibre 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.
[edit] Concealed carry sight enhancements
Specific to handguns is the issue of concealed carry. While police and military personnel generally carry their handguns in open holsters that provide easy and unobstructed access, civilians in areas where concealed carry is allowed, and who choose to carry concealed, are usually required to carry their firearms concealed under clothing or some other form of cover, such as a fanny pack or purse. The high contrast, sharp edges that are 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 for a designer of sights 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 sights of a concealed carry gun will be sloped. The rear sight won't be a vertical plate, but will have depth to 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 sights for use on a concealed handgun also have to consider the potential conditions in which they will be used--probably low light, close range, and with the operator under significant stress due to an imminent threat. Some unusual solutions have been devised, such as the trapezoidal sights (G in the open sight list above) used by Steyr, or the "gutter sight" used by the highly modified ASP S&W model 39.
[edit] Other types of sights
For precision applications such as hunting or sniping the iron sights are usually replaced by a telescopic sight, and for target shooting and combat operations low-power magnified or unmagnified optical sights such as red-dot or reflex sights are increasingly common.
[edit] Adjusting an iron sight's point of impact
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. In the illustration at right, the point of impact was LEFT and BELOW the target. To move the point of impact to the center, 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 1/2 minute clicks, then, it would take 2 clicks to move 1 inch at 100 yards, 4 clicks to move 1 inch at 50 yards, 8 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 x 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 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 x D1) /R1 = (26.25 x 5.3) / (50 x 36) = 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.
[edit] Adjusting non-adjustable sights
There are a number of ways to adjust fixed iron sights, depending on how the sights are attached to the gun.
- Dovetailed sights: These can be adjusted for windage by tapping the sights left and right in the dovetail. Front sight blades can be purchased or made with different heights, to match the performance of a particular ammo for fixing point of impact elevation issues. Many front and rear sights are dovetailed.
- Pinned, staked, or screwed in sights: These can be adjusted for elevation by replacing them with a higher or lower part. This can be done for many types of front sight.
- Revolver front sights: The front sight can be adjusted for windage by turning the barrel slightly from side to side.
- Any sights: By lowering the top of the rear sight notch, the point of impact can be lowered; by lowering the top of the front sight, it can be raised. Filing the inside left of the rear sight notch will move the point of impact left, as will filing down the right side of the front blade.
[edit] See also
[edit] References
- ^ a b c Chuck Hawks. Choosing the Right Sight.
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This article or section includes a list of references or external links, but its sources remain unclear because it lacks in-text citations. You can improve this article by introducing more precise citations. |
- Nygord's Notes on iron sights for pistol shooting
- Brownell's page on measuring for iron sights
- SightLine extension tube for increasing sight radius
[edit] External links
- Additional BRNO target sight images 1 2 3
- Choosing the Right Sight by Chuck Hawks
