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. Some even espouse a mentality that eliminates the concept of "aim" altogether. Because much of shotgunning involves putting a
scatter pattern in the path of moving targets, the sight functions mostly as a subconscious aid. The front sight of a shotgun is a small spherical "bead" attached to the
muzzle, acts as a reference, while the "rear sight" is nothing more than a narrow longitudinal groove on the
receiver and
barrel rib. When shooting, aligning the rear groove with the front bead is not to be consciously considered, as it comprises only a rough reference allowing the shooter to use their
natural point of aim to make the shot. 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 "
figure 8" 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.
Aperture sights of a
SIG SG 550 assault rifle. The viewing aperture above the "3" (denoting the 300 m setting) can be seen 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. 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.
Ghost ring 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
riot and
combat shotguns and customized
handguns, and they are also gaining ground as a backup sighting system on
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. Factory Mossberg ghost ring sights also have thick steel plates on either side of the extremely thin ring. These are to protect the sight's integrity if the shotgun were to fall and impact a surface in a way that could potentially damage or distort the shape of the ring.
Target aperture sights target diopter and globe aperture sight picture 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, of approximately or less, 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 to increments at . Some
International Shooting Sport Federation (ISSF) (Olympic) shooting events require this precision level for sighting lines, since the final score of the top competitors last shots series 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.
Front aperture size selection Front aperture size is a compromise between a tight enough aperture to clearly define the aiming point and a loose enough aperture so as to not cause 'flicker'. When the aperture is too small, the boundary between the target and front aperture outline becomes indistinct, requiring the shooter to consciously or subconsciously generate small eye movements to measure the distance around the target. USA Shooting recommends a front aperture that creates at least 3 Minutes of Angle (MOA) of boundary space. In research performed by Precision Shooting, it was found that this increased shooter confidence, reduced hold times, and created more decisive shots. There may be an upper bound to the front aperture size that improves performance, however. In 2013, researchers performed experiments with the game of golf, specifically the skill of putting which is another skill that combines visual alignment with motor skills. They found that by manipulating the perceived size of the target (the golf hole) by surrounding it with concentric rings of various sizes, there was a phenomenon that improved performance when the target was surrounded by smaller circles thereby increasing its perceived size. They found that when the target was perceived as larger, performance increased.
Non-target aperture sights ladder aperture sight calibrated out to Aperture sights on military rifles use a larger aperture with a thinner ring, and generally a simple post front sight. 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 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
vernier scales, allowing adjustment down to a single minute of arc over the full range of the sight.
Flip up sights Assault rifles and
sporterized semi-automatic rifles can have foldable rear and front sight elements that can be readily flipped up or down by the user. Such iron sights are often used as secondary sighting systems in case the main weapon sight (typically an
optical sight such as a
telescopic sight or
red dot sight) malfunctions or becomes unsuitable for the tactical situation at hand, and are therefore referred to as
backup iron sights (BUIS). Backup sights are usually mounted via
Rail Integration Systems (most often
Picatinny rails) in
tandem with optical aiming devices, although "offset" BUISs that are mounted obliquely from the
bore axis also exist. When used with non-magnifying optics (e.g.
reflex or
holographic sights), the flip-up rear and front elements often are designed to appear in the same
sight picture, known as
cowitnessing, as the primary optical sights. == Adjustment ==