真正意义上的枪械瞄准镜是于1835至1840年间发明的。根据1844年出版的《The Improved American Rifle》一书记载，旅美英国工程师约翰·R·查普曼（John Radcliffe Chapman，1815～1899）和当时著名的美国枪匠摩根·詹姆斯（Morgan James，1815～1878）两人合作设计发明了被称为“查普曼-詹姆斯瞄具”（Chapman-James sight）的望远镜式瞄具。1855年，纽约工程师威廉·马尔科姆（William Malcolm，1823～1890）将瞄准镜的设计改进并商业化，大大提高了在不同射程上的精度和适用性。马尔科姆公司设计的6倍放大瞄准镜（当时称为Hi-Lux型）和其它相似规格的瞄准镜——比如同时期佛蒙特州珠宝商L. M. Amidon生产的瞄准镜，以及同时期的大卫森和帕克戴尔瞄准镜——早在南北战争中就成为精确射手步枪上的常见配具。
- 出瞳直径（exit pupil）：光线经过目镜汇聚后，在目镜后形成的亮斑的直径。瞄准镜内部的镜片和光阑会将收集到的光过滤集中到一道光束上，这就是出瞳光。出瞳光束的直径相当于物镜直径除以放大倍数。因为任何从目镜汇聚放射出的光线必须经过人眼的瞳孔后进入视网膜成像，因此为了能最有效的让人看到最明亮清晰的图像，出瞳直径应该等于人类瞳孔的直径（白天大约为3毫米，夜晚最大可达7毫米左右）。越大的出瞳直径，给人感觉成像的亮度也越大，但大于瞳孔直径的出瞳直径会导致光束周边多余的光线都被虹膜阻拦而被“浪费”掉；而如果出瞳直径小于瞳孔直径，则会产生晕影现象使得有效视野缩水，大大增加观瞄的难度。因此，绝大多数使用者在选择瞄准镜时都会考虑选用稍微大于瞳孔直径的出瞳直径。
- 目视离隙（eye relief）：能在避免晕影清楚看见影像的情况下眼睛与目镜间所允许的最大距离，和目镜焦距有关，焦距越短目视离隙也就越短。市面上常见的望远瞄具的目视离隙可以在25毫米（0.98英寸）到100毫米（3.9英寸）以上，但是枪械上所用的瞄准镜通常为了避免观瞄时稳定困难或者目镜因为后坐力而磕碰使用者的脸和眼睛，都会采用较长的离隙。一些特殊设计的枪械，比如侦查步枪（scout rifle，比较著名的例子是斯泰尔斥候步枪），甚至会使用150毫米（5.9英寸）以上目视离隙的瞄准镜。
瞄准镜的标线种类很多，从最简单的传统十字线（crosshair）到可以用来准确估算目标距离、补算子弹下坠和风偏的复杂标尺。从制造工艺上来说，瞄准镜的标线主要有丝线式（wire reticle）和蚀刻式（etched reticle）两种。丝线式标线是最老的款式，由金属丝或布丝制成，通常安装在镜筒内特定的光学位置；蚀刻式标线则是将标线图案直接刻画到镜内的光学部件之中。如果加以背景照明，丝线式标线通常会反射一部分照明光，因此不会保持完全黑暗的高对比度；蚀刻式标线则会在背景照明下保持完全黑暗。蚀刻式标线同时可以提供更加灵活的图案设计，一些制造商甚至提供客户自己设计的特制图案，因此更加受现代使用者青睐。在高端型号和价码的瞄准镜中，通常是蚀刻式标线一统天下，但是丝线式标线仍然在低端价格的瞄准镜中占有一席之地。
许多现代瞄准镜标线在设计上都可以利用基线测距（stadiametric rangefinding）原理来估算目标距离，其中最有名也是最受欢迎的设计就是密位点式标线（mil-dot reticle）。密位点标线式在复式十字线（duplex crosshair）的基础上在距离准心每个毫弧度（密位）的间隔位置上布有一个小点，让使用者可以在知道目标已知尺寸的基础上利用密位值反向估算出大概精确的距离，并且用来补算子弹的下坠和风偏度。在北约国家的军方和警方中已经广泛使用密位点及其类似的密位类标线，并且统一使用手动调整值为0.1密位的瞄准镜，同时狙击小组中观测手与射击手之间的计算通话也以密位为单位标准。
因为瞄准镜是以折射望远镜的原理为基础设计的，因此镜内有两个聚焦平面（focal plane）可以用来放置标线：物镜与正像镜之间的第一焦平面（first focal plane，简称FFP，也称前焦面或物方焦面），以及正像镜与目镜之间的第二焦平面（second focal plane，简称SFP，也称后焦面或像方焦面）。在固定放大倍数的瞄准镜上，标线放在哪个焦平面上都无所谓；但是如果是可变倍数设计的瞄准镜，位于第一焦平面的标线会随着目标图像一起同步放大或缩小，而位于第二焦平面的标线则会忽视目标保持自己的尺寸不变。
虽然通常价码较贵，但是越来越多的追求精度的射手仍然倾向于第一焦平面的瞄准镜设计。同时为了解决第一焦平面标线在极端倍数过粗过细干扰图像的问题，现在一些厂家已经试着推出新的双焦面（dual focal plane，简称DFP）设计的高端产品，只在第一焦平面放置刻度标码，准心和十字线被挪到了第二焦平面，使用时两个焦平面的图像重叠。这种新设计的成本要远远高于普通的第一焦平面和第二焦平面产品。
Either type of reticle can be illuminated for use in low-light or daytime conditions. With any illuminated low-light reticle, it is essential that its brightness can be adjusted. A reticle that is too bright will cause glare in the operator's eye, interfering with his ability to see in low-light conditions. This is because the pupil of the human eye closes quickly upon receiving any source of light. Most illuminated reticles provide adjustable brightness settings to adjust the reticle precisely to the ambient light.
Illumination is usually provided by a battery powered LED, though other electric light sources can be used. The light is projected forward through the scope, and reflects off the back surface of the reticle. Red is the most common colour used, as it least impedes the shooter's natural night vision. This illumination method can be used to provide both daytime and low-light conditions reticle illumination.
Radioactive isotopes can also be used as a light source, to provide an illuminated reticle for low-light condition aiming. In sights like the SUSAT or Elcan C79 Optical Sight tritium-illuminated reticles are used for low-light condition aiming. Trijicon Corporation uses tritium in their combat and hunting-grade firearm optics, including the ACOG. The (radioactive) tritium light source has to be replaced every 8–12 years, since it gradually loses its brightness due to radioactive decay.
With fiber optics ambient (day)light can be collected and directed to an illuminated daytime reticle. Fiber-optics reticles automatically interact with the ambient light level that dictates the brightness of the reticle. Trijicon uses fiber optics combined with other low-light conditions illumination methods in their AccuPoint telescopic sights and some of their ACOG sights models.
Parallax problems result from the image from the objective not being coincident with the reticle. If the image is not coplanar with the reticle (that is the image of the objective is either in front of or behind the reticle), then putting the eye at different points behind the ocular causes the reticle crosshairs to appear to be at different points on the target. This optical effect causes parallax induced aiming errors that can make a telescopic sight user miss a small target at a distance for which the telescopic sight was not parallax adjusted.
To eliminate parallax induced aiming errors, telescopic sights can be equipped with a parallax compensation mechanism which basically consists of a movable optical element that enables the optical system to project the picture of objects at varying distances and the reticle crosshairs pictures together in exactly the same optical plane. There are two main methods to achieve this.
- By making the objective lens of the telescopic sight adjustable so the telescopic sight can compensate parallax errors. These models are often called AO or A/O models, for adjustable objective.
- By making an internal lens in the internal optical groups mounted somewhere in front of the reticle plane adjustable so the telescopic sight can compensate parallax errors. This method is technically more complicated to build, but generally more liked by parallax adjustable telescopic sight users—unlike AO models, which are read from the top, the sidewheel's setting can be read with minimal movement of the head. These models are often called side focus or sidewheel models.
Most telescopic sights lack parallax compensation because they can perform very acceptably without this refinement. Telescopic sights manufacturers adjust these scopes at a distance that best suits their intended usage. Typical standard factory parallax adjustment distances for hunting telescopic sights are 100 yd or 100 m to make them suited for hunting shots that rarely exceed 300 yd/m. Some long range target and military style telescopic sights without parallax compensation may be adjusted to be parallax free at ranges up to 300 yd/m to make them better suited for aiming at 300 yd/m+ ranges. Scopes for rimfire guns, shotguns, and muzzleloaders that are rarely fired at 100 yd/m+ ranges will have shorter parallax settings, commonly 50 yd/m for rimfire scopes and 100 yd/m for shotguns and muzzleloaders. Scopes for airguns that are commonly used at short ranges are very often found with adjustable parallax, usually in the form of an adjustable objective, or AO. These may adjust down as far as 3碼（2.7米）.
The reason why scopes intended for short range use are often equipped with parallax compensation is that at short range (and at high magnification) parallax errors become more noticeable. A typical scope objective has a focal length of 100 mm. An optical ideal 10× scope in this example has been perfectly parallax corrected at 1000 m and functions flawlessly at that distance. If the same scope is used at 100 m the target-picture would be projected (1000 m / 100 m) / 100 mm = 0.1 mm behind the reticle plane. At 10× magnification the error would be 10 × 0.1 mm = 1 mm at the ocular. If the same scope was used at 10 m the target-picture would be (1000 m / 10 m) / 100 mm = 1 mm projected behind the reticle plane. When magnified ten times the error would be 10 mm at the ocular.
Bullet Drop Compensation (BDC) (sometimes referred to as ballistic elevation) is a feature available on some rifle scopes. The feature compensates for the effect of gravity on the bullet at given distances (referred to as "bullet drop") in flat fire scenarios. The feature must be tuned for the particular ballistic trajectory of a particular combination of gun and cartridge at a predefined muzzle velocity and air density. Telescopic sights designed for military use like the ACOG or PSO-1 featuring BDC reticles or elevation turrets with range markings are fairly common, though commercial manufacturers offer the option to install a BDC reticle or elevation turret as long as the customer supplies the necessary ballistic data. Since the usage of standardized ammunition is an important prerequisite to match the BDC feature to the external ballistic behaviour of the employed projectiles, telescopic sights with BDC are generally intended to assist with field shooting at targets at varying medium to longer ranges rather than precise long range shots. With increasing range inevitable BDC induced errors will occur when the environmental and meteorological circumstances deviate from the predefined circumstances for which the BDC was calibrated. Marksmen can be trained to understand the main forces acting on the projectile and their effect on their particular gun and ammunition and the effects of external factors at longer ranges to counter these errors.
A telescopic sight can have several adjustment controls.
- Focusing control at the ocular end of the sight – meant to obtain a sharp picture of the object and reticle.
- Elevation or vertical adjustment control of the reticle.
- Zero-stop elevation controls can be set to prevent inadvertently dialing the adjustment knob "below" the primary zero (usually 100 meters or 100 yards for long-range scopes), or at least prevent dialing more than a couple adjustment clicks below zero. This feature is also useful on long-range scopes because it allows the shooter to physically verify the elevation knob is dialed all the way down avoiding confusion regarding the elevation status on two- or multi-revolution elevation knobs.
- Windage or horizontal adjustment control of the reticle.
- Magnification control – meant to change the magnification by turning a ring that is generally marked with several magnification power levels.
- Illumination adjustment control of the reticule – meant to regulate the brightness level of the lit parts of the reticles crosshairs.
- Parallax compensation control.
Most contemporary telescopic sights offer the first three adjustment controls. The other three are found on telescopic sights that offer a variable magnification, an illuminated reticle and/or parallax compensation. A rather common problem with the elevation and windage adjustment controls is that once smooth working adjustment turrets 'get stuck' over the years. This is generally caused by long term lack of movement in the lubricated turret mechanisms.
Older telescopic sights often did not offer windage and elevation adjustments in the scope, but rather used adjustable mounts to provide adjustment. Some modern mounts also allow for adjustment, but it is generally intended to supplement the scope adjustments. For example, some situations require fairly extreme elevation adjustments, such as very short range shooting common with airguns, or very long range shooting, where the bullet drop becomes very significant. Also, loose manufacturing tolerances may result in base mounting holes being less than perfectly aligned with the bore. In this case, rather than adjusting the scope to the extremes of its elevation adjustment, the scope mount can be adjusted. This allows the scope to operate near the center of its adjustment range, which puts less stress on the internals. Some companies offer adjustable bases, while others offer tapered bases with a given amount of elevation built in (commonly listed in MOA). The adjustable bases are more flexible, but the fixed bases are far more durable, as adjustable bases may loosen and shift under recoil. Also, adjustable bases are considerably more expensive, as well.
Typical accessories for telescopic sights are:
- Lens hoods for mounting on the objective and/or ocular to reduce/eliminate image quality impairing stray light and at the ocular secondary to avoid recoil induced facial and eye injuries.
- Lens hoods that extend the full length of a gun barrel to improve image quality by blocking out shot strings induced mirage ("heat waves" or aberrations resulting from a hot gun barrel).
- Covers to protect the objective and/or ocular external lens surface against foul weather and damage. There are slide-over, bikini and flip-open type covers without or with transparent covering material.
- Optical filters like Grey, Yellow and Polarising filters to optimize image quality in various lighting conditions.
- Kill Flash or honeycomb filters to eliminate light reflections from the objective that could compromise a sniper.
- Eye-safe laser filters to protect operators against being wounded/blinded by laser light sources. These filters are often an internal part in the assembly of lens elements.
- Transit and protection pouches and cases.
In 1997 Swarovski Optik introduced the LRS series telescopic sight, the first riflescope on the civilian market with an integrated laser rangefinder. The LRS 2-12x50 sight can measure ranges up to 600米（660 yd）. The LRS sights are currently (2008) not produced anymore, but sights with similar features are commercially available from several manufacturers.
An integrated ballistic computer/riflescope system known as BORS has been developed by the Barrett Firearms Company and became commercially available around 2007. The BORS module is in essence an electronic Bullet Drop Compensation (BDC) sensor/calculator package intended for long-range sniping out to 2,500米（2,700 yd） for some telescopic sight models made by Leupold and Nightforce. To establish the appropriate elevation setting the shooter needs to enter the ammunition type into the BORS (using touch pads on the BORS console) determine the range (either mechanically or through a laser rangefinder) and crank the elevation knob on the scope until the proper range appears in the BORS display. The BORS automatically determines the air density, as well as the cant or tilt in the rifle itself, and incorporates these environmental factors into its elevation calculations.
The SAM (Shooter-supporting Attachment Module) measures and provides aiming and ballistic relevant data and displays this to the user in the ocular of the Zeiss 6-24x72 telescopic sight it is developed for. The SAM has different sensors integrated (temperature, air pressure, shooting angle) and calculates the actual ballistic compensation. All indications are displayed in the ocular. It memorizes up to 4 different ballistics and 4 different firing tables. So it is possible to use 1 SAM with 4 total different loads or weapons without an additional adjustment.
A totally different approach has been applied in the ELCAN DigitalHunter Digital Rifle Scope series which combines CCD and LCD technology with electronic ballistics compensation, automatic video capture, 4 field selectable reticles and customizable reticles. In 2008 a DigitalHunter DayNight Riflescope that uses infrared light captured by the CCD to enhance low light capabilities became available. It is also possible to attach infrared light sources to use this telescopic sight as an active night sight in total darkness, though the image quality, and overall performance is poor. Some jurisdictions however forbid or limit to use of night vision devices for civilian or gun aiming use.
As very few firearms come with built-in telescopic sights (military designs such as the Steyr AUG, SAR 21 and the H&K G36 being exceptions) mounting a scope to a firearm requires additional equipment. Equipment is available to mount scopes on most production firearms. A typical scope mounting system consists of two parts, the scope base and the scope rings. By picking the appropriate combination of scope base to fit the firearm and scope rings to fit the scope, a wide range of scopes may be mounted to most firearms. With the appropriate combination of adjustable scope bases and scope rings it is also possible to mount several telescopic sights on the same gun to make the gun more versatile. However, it is important to take into consideration whether or not a gun is particularly hard to mount. If it is or if a gun is intended for long-range shooting, it could be that the amount of vertical adjustment range is smaller than required. This can be solved with the help of a vertically canted base or canted rings. Typical cant angles offered by mounting components manufacturers are 20 and 30 MOA.
The base is attached to the rifle, usually with screws, and is often designed to have a low profile, and to allow use of the iron sights if the scope is not present. Some manufacturers provide integral bases on many of their firearms; an example of such a firearm is the Ruger Super Redhawk revolver. The most commonly encountered mounting systems are the 3/8 inch (9.5 mm) and the 11 mm dovetail mounts (sometimes called tip-off mounts), commonly found on rimfires and air guns, the Weaver type base and the STANAG 2324 (MIL-STD-1913 "Picatinny rail") base. Ruger uses a proprietary scope base system, though adapters are available to convert the Ruger bases into Weaver type bases. Scope base and mounting systems are also manufactured in Europe. Specialized manufacturers like Ernst Apel GmbH offer an elaborate program of mounting solutions for many different guns. Some of the European mounting solutions are virtually unknown and hence rarely applied in America. Many European gun manufacturers also developed and offer proprietary scope base systems for their guns, for example Sako has tapered dovetails and Tikka uses 16mm dovetail.
In addition to needing the right type of connector to attach to the desired base, scope rings must be used to hold the scope to the mount. The rings must be of the proper size to fit the scope; common sizes are .75英寸（19.05 mm）, 22 mm（0.87英寸）, 1英寸（25.40 mm）, 26 mm（1.02英寸）, 30 mm（1.18英寸）, 34 mm（1.34英寸） and 40 mm（1.57英寸）. Red dot sights commonly are found in larger sizes, such as 40 mm, and these often use ringless mounting systems designed to fit dovetail or Weaver type bases. Rings are also available in a variety of heights and materials. Ring height is chosen to place the scope high enough to clear the firearm, and at a height comfortable for the shooter.
European telescopic sight manufacturers often offer the option to have mounting rails underneath the riflescope to provide for mounting solutions that do not use scope rings or a single scope ring around the objective of the scope. These rails are an integral part of the scope body and can not be removed. The mounting rail permits the riflescope to be securely and tension-free mounted at the preferred height and correct distance from the shooter's eye and on different guns.
There are several mounting rail systems offered:
The traditional standard prism mounting rail system requires to have the scope rail drilled from the side for fixture screws. The more recent propriety systems mainly offer aesthetic advantages for people who have problems with redundant drill holes in sight in case the riflescope is used on different guns. To avoid drilling the scope rail, the propriety rail mounting systems have special shape connections machined in the inside of the rail. These shape connections prevent ever showing any exterior damage from mounting work on the rifle scope. The propriety rail systems use matching slide-in mount fasteners to connect the riflescope to the gun. Some propriety rails also offer the possibility to tilt the scope up to 1° to the left or right.
For mounting telescopic sights and/or other accessories to guns several rail interface systems are available to provide a standardized mounting platform. Probably the best known rail interface system is the Picatinny rail or STANAG 2324 rail or MIL-STD-1913 rail used by NATO forces and other official and civil users. The name of this interface system, which dates back to 3 February 1995, comes from the Picatinny Arsenal in New Jersey, where it was originally tested and was used to distinguish it from other rail standards at the time. The Picatinny rail comprises a series of ridges with a T-shaped cross-section interspersed with flat "spacing slots". Telescopic sight mounting rings are mounted either by sliding them on from one end or the other; by means of a "rail-grabber" which is clamped to the rail with bolts, thumbscrews or levers; or onto the slots between the raised sections.
Another commercially available rail interface system is the Weaver rail mount from Weaver Optics. The only difference between the Picatinny rail and the Weaver rail is the size and spacing of the slots, although almost all rail-grabber-mounted accessories are manufactured such that they can mounted on either type of rail.
The NATO Accessory Rail (or NAR), defined by the new modernization agreement STANAG 4694 approved by NATO on 8 May 2009, is a new rail interface system standard for mounting auxiliary equipment such as telescopic sights, tactical lights, laser aiming modules, night vision devices, reflex sights, foregrips, bipods, and bayonets to small arms such as rifles and pistols. The NATO Accessory Rail is backwards-compatible with the STANAG 2324 or MIL-STD 1913 Picatinny rail.
Scopes for use on light-recoiling firearms, such as rimfire guns, can be mounted with a single ring, and this method is not uncommon on handguns, where space is at a premium. Most scopes are mounted with two rings, one in the front half of the scope and one on the back half, which provides additional strength and support. The heaviest-recoiling firearms, such as Thompson Center Arms Contender pistols in heavy-recoiling calibers, will use three rings for maximum support of the scope. Use of too few rings can result not only in the scope moving under recoil, but also excessive torque on the scope tube as the gun rolls up under recoil.
Scopes on heavy-recoiling firearms and spring piston airguns (which have a heavy "reverse recoil" caused by the piston reaching the end of its travel) suffer from a condition called scope creep, where the inertia of the scope holds it still as the firearm recoils under it. Because of this, scope rings must be precisely fitted to the scope, and tightened very consistently to provide maximum hold without putting uneven stress on the body of the scope. Rings that are out of round, misaligned in the bases, or tightened unevenly can warp or crush the body of the scope.
Another problem is mounting a scope on a rifle where the shell is ejected out the top of the action, such as some lever action designs. Usually this results in the scope being offset to one side (to the left for right-handed people, right for left-handed) to allow the shell to clear the scope. Alternately a scout rifle type mount can be used, which places a long-eye-relief scope forward of the action.
A firearm may not always be able to fit all aiming optics solutions, so it is wise to have a preferred aiming optics solution first reviewed by a professional.
Telescopic sights have both advantages and disadvantages relative to iron sights. Standard doctrine with iron sights is to focus the eye on the front sight and align it with the resulting blur of the target and the rear sight; most shooters have difficulty doing this, as the eye tends to be drawn to the target, blurring both sights. Gun users over 30 years of age with keen eyesight will find it harder to keep the target, front sight element and rear sight element in focus well enough for aiming purposes, as human eyes gradually lose focusing flexibility with rising age, due to presbyopia. Telescopic sights allow the user to focus on both the crosshair and the target at the same time, as the lenses project the crosshair into the distance (50 m or yd for rimfire scopes, 100 m or yd more for centerfire calibers). This, combined with telescopic magnification, clarifies the target and makes it stand out against the background. The main disadvantage of magnification is that the area to either side of the target is obscured by the tube of the sight. The higher the magnification, the narrower the field of view in the sight, and the more area is hidden. Rapid fire target shooters use reflex sights, which have no magnification; this gives them the best field of view while maintaining the single focal plane of a telescopic sight. Telescopic sights are expensive and require additional training to align. Sight alignment with telescopic sights is a matter of making the field of vision circular to minimize parallax error. For maximum effective light-gathering and brightest image, the exit pupil should equal the diameter of the fully dilated iris of the human eye — about 7 mm, reducing with age.
Though they had been used as early as the 1850s on rifles, and even earlier for other tasks, until the 1980s, when optical device and assault rifle combinations such as the Austrian Steyr AUG and the British SUSAT mounted on the SA80, became standard issue, military use of telescopic sights was restricted to snipers because of the fragility and expense of optical components. Additionally the glass lenses are prone to breakage, and environmental conditions such as condensation, precipitation, dirt, and mud obscure external lenses. The scope tube also adds significant bulk to the rifle. Snipers generally used moderate to high magnification scopes with special reticles that allow them to estimate range to the target. Since the 1990s many other armed forces have adopted optical devices for general issue to infantry units and the rate of adoption has increased as the cost of manufacture has fallen.
Telescopic sights provide some tactical disadvantages. Snipers rely on stealth and concealment to get close to their target. A telescopic sight can hinder this because sunlight may reflect from the lens and a sniper raising his head to use a telescopic sight might reveal his position. The famous Finnish sniper Simo Häyhä preferred to use iron sights rather than telescopic sights to present less of a target. Harsh climate can also cause problems for telescopic sights as they are less rugged than iron sights. Many Finnish snipers in WWII used iron sights heavily because telescopic sights did not cope with very cold Finnish winters.
The market for military telescopic sights intended for military long-range shooting is highly competitive. Several high end optics manufacturers are constantly adapting and improving their telescopic sights to fulfill specific demands of military organizations. Two European companies that are active this field are Schmidt & Bender and Zeiss/Hensoldt. American companies that are also very active in this field are Nightforce, U.S. Optics Inc. and Leupold. These high end sighting components generally cost €1500 / $2000 or more. Typical options for military telescopic sights are reticle illumination for use under adverse light circumstances and the presentation of scope settings or ballistic relevant environmental measurements data to the operator through the sights ocular.
The former Warsaw Pact members produce military telescopic sights for their designated marksmen and developed a range finding reticle based on the height of an average human. This stadiametric rangefinder reticle was originally used in the Russian PSO-1 4×24 rifle scope and is calibrated for ranging a 1.7 m tall target from 200 m to 1000 m. The target base has to be lined up on the horizontal line of the range-finding scale and the target top point has to touch the upper (dotted) line of the scale without clearance. The digit under which this line up occurs determines the distance to the target. The PSO-1 basic design and stadiametric rangefinder are also found in the POSP and other telescopic sight models.
The Israeli military began widespread use of telescopic sights by ordinary infantrymen to increase hit probability (especially in dim light) and extend effective range of standard issue infantry rifles. Palestinian militants in the al Aqsa Intifada likewise found that adding an inexpensive scope to an AK-47 increased its effectiveness.
Today, several militaries issue telescopic sights to their infantry, usually compact, low-magnification sights suitable for snap-shooting. The US military issues the Advanced Combat Optical Gunsight (ACOG), designed to be used on the M16 rifle and M4 carbine. American soldiers in Iraq and Afghanistan frequently purchase their own combat optics and carry them from home. The British army fields the SA80 rifle with the SUSAT 4× optical sight as standard issue. The Canadian Forces standard C7 rifle has a 3.4× Elcan C79 optical sight. Both Austria and Australia field variants of the Austrian Steyr AUG which has built an integral 1.5× optical sight since its deployment in the late 1970s. The German Army G36 assault rifles have a more or less built in dual combat sighting system consisting of a ZF 3x4° telescopic sight combined with an unmagnified electronic red dot sight. The dual combat sighting system weighs 30 g（1.1 oz） due to a housing made out of glass fibre reinforced polyamide. All German G36 rifles are adapted to use the Hensoldt NSA 80 II third-generation night sight, which clamps into the G36 carry handle adapter in front of the optical sight housing and mates with the rifle's standard dual combat sighting system.
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- High-speed video of a Schmidt & Bender PMII telescopic sight, mounting and barrel flexing on a .50 BMG Accuracy International AS50.
- 2014 field test focused on long-range, tactical rifle scopes in the $1,500+ price range