
Telescopic sight

A telescopic sight, commonly called a scope for short, is an optical sighting device based on a refracting telescope. It is equipped with some form of a referencing pattern – known as a "reticle" – mounted in a focally appropriate position in its optical system to provide an accurate point of aim. Telescopic sights are used with all types of systems that require magnification in addition to reliable visual aiming, as opposed to non-magnifying iron sights, reflector (reflex) sights, holographic sights or laser sights, and are most commonly found on firearms, particularly rifles. The optical components may be combined with optoelectronics to form a digital night scope or a "smart" scope. Scope sights are usually attached to a firearm with a scope mount.
History.
The first experiments directed to give shooters optical aiming aids go back to the early 17th century. For centuries different optical aiming aids and primitive predecessors of telescopic sights were created that had practical or performance limitations.
In the late 1630s, the English amateur astronomer William Gascoigne was experimenting with a Keplerian telescope and left it with the case open. Later he found that a spider had spun its web inside the case and when he looked through the telescope found that the web was in focus as well as distant objects and he realised that he could use this principle to make a telescopic sight for use in his astronomical observations.
In 1776, Charles Willson Peale attempted to have a telescope mounted to a rifle as a sighting aid, but was unable to mount it sufficiently far forward to prevent impact with the operator's eye on recoil.
The first documented telescopic rifle sight was invented between 1835 and 1840. In a book titled "The Improved American Rifle", written in 1844, civil engineer John R. Chapman documented the first telescopic sights made by Morgan James of Utica, New York. Chapman gave James the concepts and some of the design, whereupon they produced the Chapman-James sight. In 1855, William Malcolm of Syracuse, New York began producing his own sight. Malcolm used an original design incorporating achromatic lenses like those used in telescopes, and improved the windage and elevation adjustments. They were between three and twenty magnification (possibly more). Malcolm's and those made by L. M. Amidon of Vermont were the standard during the Civil War.
Still other telescopic rifle sights of the same period were the Davidson and the Parker Hale.
An early practical refractor telescope based telescopic sight was built in 1880 by August Fiedler (Stronsdorf, Austria), forestry commissioner of Prince Reuss.
Later telescopic sights with extra long eye relief became available for handgun and scout rifle use. A historic example of a telescopic sight with a long eye relief is the German ZF41 which was used during World War II on Karabiner 98k rifles.
An early example of a man portable telescopic sight for low visibility/night use is the "Zielgerät" (aiming device) 1229 (ZG 1229), also known by its code name Vampir. The ZG 1229 Vampir was a Generation 0 active infrared night vision device developed for the "Wehrmacht" for the StG 44 assault rifle, intended primarily for night use. The issuing of the ZG 1229 Vampir system to the military started in 1944 and it was used on a small scale in combat from February 1945 until the final stages of World War II.
Types.
Telescopic sights are classified in terms of the optical magnification (i.e. "power") and the objective lens diameter. For example, "10×50" would denote a magnification factor of 10, with a 50 mm objective lens. In general terms, larger objective lens diameters, due to their ability to gather a higher luminous flux, provide a larger exit pupil and hence provide a brighter image at the eyepiece. 
There are also telescopic sights with variable magnification. The magnification can be varied by manually operating a zoom mechanism. Variable-power sights offer more flexibility regarding shooting at varying ranges, targets and light conditions and offer a relative wide field of view at lower magnification settings. The syntax for variable sights is the following: "minimal magnification – maximum magnification × objective lens", for example "3-9×40" means a telescopic sight with variable magnification factor between 3 and 9, and a 40 mm objective lens. Variable-power telescopic sights in the low magnification range (1-4×, 1-6× or 1-8×, even 1-10×) are informally referred to as "low-power variable optics" (LPVO). 
Confusingly, some older telescopic sights, mainly of German or other European manufacture, have a different classification where the second part of the designation refers to 'light gathering power.' In these cases, a 4×81 (4× magnification) sight would be presumed to have a brighter sight picture than a 2.5×70 (2.5× magnification), but the objective lens diameter would not bear any direct relation to picture brightness, as brightness is affected also by the magnification factor. Typically objective lenses on early sights are smaller than modern sights, in these examples the 4×81 would have an objective 36 mm diameter and the 2.5×70 should be approximately 21 mm (relative luminosity is the square of the exit pupil as measured in mm; a 36 mm objective lens diameter divided by the 4× magnification gives an exit pupil of 9 mm; (9×9=81)
A relatively new type telescopic sight, the prismatic telescopic sight or prism scope, does away with the image erecting lens system of a traditional telescopic sight by replacing it with a prism image erector (similar to what is found in binoculars and spotting scopes). The reticle for this type of sight is etched onto one of the prism's reflection surfaces. This allows an easy way to illuminate the reticle (from the back side of the prism) and presents the viewer with a reticle even when the illumination is turned off. Since its an optical telescope, prism scopes can compensate for a user's astigmatism and, when used as a 1x sight configuration, are an alternative to reflector sights. One example of a well known series of prismatic sights is the Trijicon ACOG.
Optical parameters.
Telescopic sights are usually designed for the specific application for which they are intended. Those different designs create certain optical parameters. Those parameters are:
Main tube.
The main tube of telescopic sights vary in size, material, the applied production process and surface finish. The typical outer diameters vary between and . The internal diameter of the telescopic sight main tube influences the area light can pass through, lens elements and other parts can be mounted in and the amount the internal parts for elevation and windage adjustment can move. Telescopic sights intended for long range and/or low light usage generally feature larger main tube diameters. Besides optical, spatial and attainable range of elevation and windage adjustments considerations, larger diameter main tubes offer the possibility to increase the tube walls thickness without sacrificing a lot of internal diameter.
Adjustment controls.
A telescopic sight can have several adjustment controls.
All telescopic sights have the first three (diopter, elevation, windage) adjustment controls, and the fourth (magnification) control is offered on variable-power scopes. The remaining two adjustments are optional and typically only found on higher-end models with additional features.
The windage and elevation adjustment knobs (colloquially called "turrets") often have internal ball detents to help accurately index their rotation, which provide a crisp tactile feedback corresponding to each graduation of turn, often accompanied by a soft but audible clicking sound. Each indexing increment is thus colloquially called a "click", and the corresponding angular adjustment of the optical axis is known as the "click value". The most commonly seen click values are  MOA (often expressed in approximations as " inch at 100 yards") and 0.1 mil (often expressed as "10 mm at 100 meters"), although other click values such as  MOA,  MOA or  MOA and other mil increments can also be infrequently seen on the commercial and military and law enforcement markets. A common problem with the elevation and windage adjustments is that formerly smooth-working turrets can "get stuck" over the years, and is generally caused by long-term lack of movement in the lubricated turret mechanisms, which caused the ball detents to gall and stagnate.
Older telescopic sights often did not offer internal windage and/or elevation adjustments in the telescopic sight. In case the telescopic sight lacked internal adjustment mechanisms adjustable mounts are used (on the scope rings, or the mounting rail itself) for sighting-in.
Reticles.
Telescopic sights come with a variety of different reticles, ranging from the simple crosshairs to complex reticles designed to allow the shooter to range a target, to compensate for the bullet drop and windage required due to crosswinds. A user can estimate the range to objects of known size, the size of objects at known distances, and even roughly compensate for both bullet drop and wind drifts at known ranges with a reticle-equipped scope.
For example, with a typical Leupold brand duplex 16 minute of angle (MOA) reticle (of a type as shown in image B) on a fixed-power scope, the distance from post to post (that is, between the heavy lines of the reticle spanning the center of the scope picture) is approximately at , or, equivalently, approximately from the center to any post at 200 yards. If a target of a known diameter of 16 inches fills just half of the total post-to-post distance (i.e. filling from scope center to post), then the distance to target is approximately . With a target of a diameter of 16 inches that fills the entire sight picture from post to post, the range is approximately 100 yards. Other ranges can be similarly estimated accurately in an analog fashion for known target sizes through proportionality calculations. Holdover, for estimating vertical point of aim offset required for bullet drop compensation on level terrain, and horizontal windage offset (for estimating side to side point of aim offsets required for wind effect corrections) can similarly be compensated for through using approximations based on the wind speed (from observing flags or other objects) by a trained user through using the reticle marks. The less-commonly used holdunder, used for shooting on sloping terrain, can even be estimated by an appropriately-skilled user with a reticle-equipped scope, once both the slope of the terrain and the slant range to target are known.
There are two main types of reticles:
Wire reticles are the oldest type of reticles and are made out of metal wire or thread. They are mounted in an optically appropriate position in the telescopic sight's tube. Etched reticles are images of the desired reticle layout that are etched on an optic element. This optical element (lens) with the etched reticle is then mounted in the telescopic sights tube as an integrated part of the optics chain of the sight. When backlit through the ocular, a wire reticle will reflect incoming light and not present a fully opaque (black) reticle with high contrast. An etched reticle will stay fully opaque (black) if backlit. Etched reticles are considered by most to be a more refined solution and offer greater reticle layout flexibility. Because of this, some manufacturers can provide client-designed custom reticles on special order. In the more expensive and high-end contemporary telescopic sights, etched reticles dominate the market. In cheaper telescopic sights, wire reticles are still often mounted to avoid a rather specialized and costly production step.
Mil-dot reticles.
Many modern reticles are designed for (stadiametric) rangefinding purposes. Perhaps the most popular and well-known ranging reticle is the "Mil-dot" reticle, which consists of duplex crosshairs with small dots at milliradian (mil, a thousandth of a radian) intervals in the field of view. This reticle has gained significant acceptance in NATO and other military and law enforcement organizations. Training and practice will enable a user to measure the range to objects of known size, the size of objects at known distances, and compensate for both bullet drop and wind drift at known ranges with reasonable accuracy. To promote methodological uniformity, (mental) calculations and communication between spotters and snipers in sniper teams, the elevation or vertical adjustment and windage controls of Mil-dot reticle equipped telescopic sights are usually adjustable in (decimal) 0.1 milliradian increments. There are, however, (military) Mil-dot equipped telescopic sights that use coarser or finer reticle adjustment increments.
By means of a mathematical formula–(width or height of the target/ number of mil of dots) × 1000 = distance—the user can measure the range to a target. An object 1 meter tall or wide is exactly 1 Mil tall or wide at 1000 meters' distance. If the user sees an object of 1.8 m tall, for example, as three mil dots tall through the riflescope the object is at 600 m distance—(1.8 / 3) × 1000 = 600.
Reticle focal plane.
Telescopic sights based on image erector lenses (used to present to the user with an upright image) have two planes of focus where a reticle can be placed: at the focal plane between the objective and the image erector lens system (the First Focal Plane (FFP)), or the focal plane between the image erector lens system and the eyepiece (the Second Focal Plane (SFP)). On fixed power telescopic sights there is no significant difference, but on variable power telescopic sights a first focal plane reticle expands and shrinks along with the rest of the image as the magnification is adjusted, while a second focal plane reticle would appear the same size and shape to the user as the target image grows and shrinks. In general, the majority of modern variable-power scopes are SFP unless stated otherwise. Every European high-end telescopic sight manufacturer offers FFP reticles on variable power telescopic sights, since the optical needs of European hunters who live in jurisdictions that allow hunting at dusk, night and dawn differ from hunters who traditionally or by legislation do not hunt in low light conditions.
The main disadvantage of SFP designs comes with the use of range-finding reticles such as mil-dot. Since the proportion between the reticle and the target is dependent on selected magnification, such reticles only work properly at one magnification level, typically the highest power. Some long-range shooters and military snipers use fixed-power scopes to eliminate this potential for error. Some SFP scopes take advantage of this aspect by having the shooter adjust magnification until the target fits a certain way inside the reticle and then extrapolate the range based on the power adjustment. Some Leupold hunting scopes with duplex reticles allow range estimation to a White-tailed deer buck by adjusting magnification until the area between the backbone and the brisket fits between the crosshairs and the top thick post of the reticle. Once that's done, the range be read from the scale printed on the magnification adjustment ring.
Although FFP designs are not susceptible to magnification-induced errors, they have their own disadvantages. It's challenging to design a reticle that is visible through the entire range of magnification: a reticle that looks fine and crisp at 24× magnification may be very difficult to see at 6×. On the other hand, a reticle that's easy to see at 6× may be too thick at 24× to make precision shots. Shooting in low light conditions also tends to require either illumination or a bold reticle, along with lower magnification to maximize light gathering. In practice, these issues tend to significantly reduce the available magnification range on FFP scopes compared to SFP, and FFP scopes are much more expensive compared to SFP models of similar quality. Most high-end optics manufacturers leave the choice between a FFP or SFP mounted reticle to the customer or have scope product models with both setups.
Variable-power telescopic sights with FFP reticles have no problems with point of impact shifts. Variable-power telescopic sights with SFP reticles can have slight point-of-impact shifts through their magnification range, caused by the positioning of the reticle in the mechanical zoom mechanism in the rear part of the telescopic sight. Normally these impact shifts are insignificant, but accuracy-oriented users, who wish to use their telescopic sight trouble-free at several magnification levels, often opt for FFP reticles. Around the year 2005 Zeiss was the first high-end European telescopic sight manufacturer who brought out variable magnification military grade telescopic sight models with rear SFP mounted reticles. They get around impermissible impact shifts by laboriously hand-adjusting every military grade telescopic sight. The American high-end telescopic sight manufacturer U.S. Optics Inc. also offers variable magnification military grade telescopic sight models with SFP mounted reticles.
Reticle illumination.
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 their 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.
Additional features.
Bullet drop compensation.
Bullet drop compensation (BDC, sometimes referred alternatively as "ballistic elevation") is a feature available on some telescopic sights, usually those used by more tactically-oriented semi-automatic/assault rifles. The feature provides pre-determined reference markings for various distances (referred to as "bullet drops") on the reticle or (much less commonly) on the elevation turret, which gives reasonably accurate estimations of potential gravitational deviation upon the bullet in flat-firing scenarios, so the shooter can proactively adjust their aim to compensate without needing to trial with missed shots or dealing with complex ballistic calculation. The BDC feature is usually tuned only for the ballistic trajectory of a particular gun-cartridge combination with a predefined projectile weight/type, muzzle velocity and air density. Military scopes featuring BDC reticles (e.g. the ACOG) or elevation turrets with range markings (e.g. PSO-1) are fairly common, though commercial manufacturers also 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 within varying medium to longer ranges rather than precise long range shooting. 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.
Parallax compensation.
Parallax problems result from the target image projected from the objective not being coplanar with the reticle. If the target and the reticle are not coplanar (i.e. focal plane of the target is either in front of or behind the reticle), when the shooter's pupil position changes (often due to small alterations in head alignment) behind the eyepiece, the target will produce a different parallax to the reticle image. This parallax difference will produce an apparent movement of the reticle "floating" over the target, known as the "parallax shift". This optical effect causes aiming errors that can make a shooter miss a small target at a distance, because he/she is actually aiming at a different spot to the assumed point of aim. It can also lead to unreliabilities when zeroing the gun.
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 can shift the target/reticle focus back or forward into exactly the same optical plane. There are two main methods to achieve this.
Most telescopic sights lack parallax compensation due to cost-benefit, as they can perform very acceptably without such refinement since most applications do not demand very high precision, so adding extra production cost for parallax compensation is not justified. For example, in most hunting situations, the "kill zone" on the game (where the vital organs are located) can be so forgivingly big that a shot hitting anywhere within the upper torso guarantees a successful kill. In these scopes, the manufacturers often design for a "parallax-free" distance that best suits their intended usage. Typical standard parallax-free distances for hunting telescopic sights are or as most sport hunting rarely exceed 300 yd/m. Some long-range target and "tactical-style" scopes without parallax compensation may be adjusted to be parallax-free at ranges up to 300 yd/m to make them better suited for the longer ranges. Telescopic sights used by rimfire guns, shotguns and muzzleloaders that are rarely fired beyond 100 yd/m ranges will have shorter parallax settings, commonly 50 yd/m for rimfire scopes and 100 yd/m for shotguns and muzzleloaders. However, due to parallax effect being more pronounced at close distances (as a result of foreshortening), scopes for airguns (which are commonly used at very short ranges) almost always have parallax compensation, frequently an adjustable objective design, which may adjust down to as near as .
The reason why telescopic sights intended for short range use are often equipped with parallax compensation is that at short range (and at high magnification) parallax errors become proportionally more noticeable. A typical telescopic sight objective lens has a focal length of . An optically ideal 10× scope in this example has been perfectly parallax corrected at and functions flawlessly at that distance. If the same scope is used at 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 telescopic sight was used at the target picture would be (1000 m / 10 m) / 100 mm = 1 mm projected behind the reticle plane. When 10× magnified the error would be 10 × 1 mm = 10 mm at the ocular.
Accessories.
Typical accessories for telescopic sights are:
Optronic technologies.
Integrated laser rangefinder.
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 . The LRS sights are currently (2008) not produced anymore, but sights with similar features are commercially available from several manufacturers.
Ballistic support devices.
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 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 6-24×72 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 four total different loads or weapons without an additional adjustment.
CCD and LCD technology.
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.
Mounting.
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.
Scope bases.
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 rails (sometimes called tip-off mounts), commonly found on rimfires and air guns, the Weaver type base, the STANAG 2324 (MIL-STD-1913 "Picatinny rail") base, and the STANAG 4694 NATO Accessory Rail. Ruger uses a proprietary scope base system, though adapters are available to convert the Ruger bases into Weaver type bases.
Scope ring sizes.
In addition to needing the right type of connector to attach to the desired base on the firearm (e.g. a picatinny rail), a scope mount needs to have a way to mount to the scope. Ring mounting is the most common method, and the ring size must be chosen according to the size of the outer main tube of the optic. Scopes with a larger main tube have more room for the erector assembly, thereby allowing for a design with increased elevation adjustment.
The three most common standards are:
Scope mounting rails.
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 telescopic sight 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 proprietary 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 proprietary 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 proprietary rail systems use matching slide-in mount fasteners to connect the rifle scope to the gun. Some proprietary rails also offer the possibility to tilt the scope up to 1° (60 moa; 17.5 mrad) to the left or right.
Technical advantages of rail mounting systems are the reliability and robustness of such mounting solutions. Even under hard recoil there will be no play in mounts and tolerances will not change over time and hard use. The additional material due to rail on the underside of the scope construction also adds stiffness and robustness to the scope body.
Rail interface systems.
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.
Mounting issues.
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.
Adjustable mounts.
Some modern mounts also allow for adjustment, but it is generally intended to supplement the telescopic sight's own internal adjustments in the case of needing unusually large elevation 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 and thus requires more elevation compensation than the sight internal adjusment mechanism can provide. 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 telescopic sight to the extremes of its elevation adjustment, the telescopic sight mount can be adjusted. This allows the telescopic sight to operate near the center of its adjustment range, which puts less stress on the internal componments. 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.
Uses.
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 meters or yards for rimfire scopes, 100 meters or yards 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.
Military.
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 U.S. 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 3×4° telescopic sight combined with an unmagnified electronic red dot sight. The dual combat sighting system weighs 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.