Iron sight

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Iron sights are a system of aligned markers used as a sighting device to assist in the aiming of a device such as a firearm, crossbow, or telescope. They are sometimes used along aiming optics such as 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 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.

Adjustable sights

rimfire rifle, with calibrated markings for ranges out to 300 meters]]

Many iron sights are designed to be adjustable, so that the sights can be adjusted for windage and elevation. In addition, adjustable sights allow compensation 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 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.

Types of 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.

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 consisting of a square or rectangular post and a flat-bottomed square notch are the most common form of open sights and are 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. 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'". 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. 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 will not 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 used by Steyr (shown as G in Types of Iron Sights), or the "gutter sight" used by the highly modified ASP S&W; model 39.

Optical 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 reflector (reflex) sights are increasingly common. Iron Sights may still be fitted alongside a telescopic sight and are referred to as Back Up Iron Sights (Acronym B.U.I.S), typically BUIS sights fold down when not in use.

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.

See also

List of telescope parts and construction

Telescopic sight

Reflex sight

Laser sight

References

Notes

External links

Additional BRNO target sight images 12 3

Category:Firearm components Category:Firearm sights Category:Artillery components