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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.〔(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 component sights, formed by metal blades: a rear sight mounted perpendicular to the line of sight and a front sight that is a post, bead, or ring. ''Open sights'' use a notch of some sort as the rear sight, while ''aperture sights'' use some form of a circular hole. Civilian, hunting, and police firearms usually feature open sights, while many military battle rifles employ aperture sights. 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. On many firearms the rear sight is adjustable for elevation or windage.〔Hawks, Chuck. ("Choosing the Right Sight" ), Chuck Hawks Web site. Retrieved July 24, 2008.〕 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 other sighting devices (or in the case of some models of optics, incorporated integrally) for back-up usage. == Theory == In the case of firearms, where the projectile follows a Newtonian trajectory, front and rear sights must be aligned with the line of sight of the shooter to the target, known as the 'Point of Aim' (POA), calibrated to the distance of the target and the trajectory of the bullet, so that the bullet hits the target at the 'Point of Impact' (POI). 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 and if the hood is circular, then this provides a reference where the eye will naturally align one within the other.〔 With typical blade or post iron sights, the shooter would 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 a 10 metre 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 , 1500 times the sight misalignment.〔Calculations assume a sight radius or sighting line〕 A long sighting line, called the 'sight radius', 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 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 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Iron sights」の詳細全文を読む スポンサード リンク
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