Scope mount
Scope mounts are used to attach telescopic sights or other types of sights to firearms. The scope sight itself is usually made for only one of two main types of mounts, which can be classified as "scopes for ring mounts" (for example a 30 mm tube) or "scopes for rail mounts" (like for example the Zeiss rail). Words such as "mounts" and "bases" are used somewhat loosely, and can refer to several different parts which are either used together or in place of each other as ways to mount optical sights to firearms. When it comes to the  interface of the firearm itself, the Picatinny rail is one of the most widespread standard for new firearms as of 2020. While most scopes are made for being mounted either with a "ring mount" or a "rail mount", some sights have an integral mounting mechanism allowing them to be attached directly to the firearm, like for example an integrated Picatinny mount. In addition, there are many proprietary and brand-specific types of mounts that either can be used with Picatinny rails or as alternatives to Picatinny (see the section on Link between scope and firearm). Scope mounts may be offered  by firearm and scope manufacturers, or on the aftermarket. Scopes for rail mounts. Zeiss rail. Among scopes for rail mounts, the 22.5 degree V-shaped Zeiss rail is the most prevalent standard. It was introduced in 1990, and after the patent expired in 2008, compatible scopes have been offered from manufacturers such as Blaser, Leica, Minox, Meopta, Nikon, Noblex (formerly Docter), Schmidt & Bender and Steiner. It has therefore, in some sense, become the "de facto" industry standard for scope mounting rails. The system has so far seen most use on the European high end market. Swarovski SR rail. The Swarovski  SR rail (patented in 2002, introduced in 2005 The Swarovski SR rail is also used by Kahles, a Swarovski subsidiary.) has a flat rail with many "teeth" as recoil lugs, and is only offered on scopes from Swarovski and its subsidiary Kahles. It separates itself from the Zeiss rail in that it is not neither stepless nor self-centering. S&B Convex rail. A former competing standard was the halv-circle shaped Schmidt & Bender Convex rail (also introduced in 2005), but Schmidt & Bender after a few years changed to following the Zeiss rail standard. In contrast to the Zeiss and Swarovski  systems, the S&B Convex rail had the possibility to add a cant to the scope when mounting (such that the reticle is not horizontal to the ground). 70 degree prism rail. There is also an older European system with an upside-down V-shape (70 degrees), but this system has little widespread use today. The advantage of this system was that it at one time was offered by most European scope manufacturers, but the disadvantage was that the rail had to be drilled for a screw each time the eye relief was to be adjusted. All new standards for  rail mounts have addressed this issue. Scopes for ring mounts. Ring mounts usually consist of a base attached to the firearm and rings (usually two) attached to the sight. The rings are usually made of steel or aluminum. Common diameters on ring mounts are 25.4 mm (1 inch), 26 mm, 30 mm and 34 mm. There are big differences in the strength and ability of sustained precision on different assemblies. With weak cartridges such as .22 LR applied in light-use scanarios, a pair of skinny aluminium rings may work well, while firearms with very powerful recoil often combined with  a heavy sight may require steel rings or thicker aluminum rings with recoil lugs to be used. Sizes. Scopes for ring mounts are available in many different sizes. The most common ones are: Some less common standards are: Lapping. In order for a ring assembly to grip evenly, it is important that the scope rings are circular and coaxial with the scope tube. On ring mounts that grip unevenly, the ring mount can be lapped to prevent uneven pressure when mounting. One scopes made for ring mounts, it is not uncommon to get ring marks when mounting the rings. Ring  inserts. There are insert rings on the market which allows for mounting a scope inside a ring mount of a larger diameter. An example could be to mount a scope with a 1-inch (25.4 mm) tube in a 30 mm mount using a plastic insert. There are also special ring mounts in the market with circularly shaped ring inserts made to provide stress free mounting without lapping, with Burris Signature Rings and Sako Optilock Rings as two well-known examples. Burris Signature was introduced in 1995. A patent was applied for in 1994, and was granted in 1995. Sako  Optilock has been sold since some time in the early 2000s. The trade name Optilock was registered in USA on 31 December 1997, and is listed as having been marketed in USA since 31 December 2001. In 2000, Sako was sold to Beretta Holding. In 2002, Burris was also sold to Beretta Holding, and thus Burris and Sako got the same owners. Burris' original patent for the rings with the circular insertes was considered to have expired in 2014, and as of 2020 is listed as "definitely expired". In 2015, "XTR Signature Rings" was launched as a further development of  the Burris Signature series. The XTR variant differs in that it has two circular cavities per ring assembly versus one. A patent for the XTR Signature Rings was applied for in 2016, and was granted to Burris in 2019. Link between scope and firearm. Bases. By "bases," is usually meant an interconnecting part between the scope and the firearm. For example, a base may have a picatinny attachment on the underside, while the upper side may have either a ring (e.g. 30 mm) or rail mounting (e.g. Zeiss rail). On some assemblies, the upper and lower parts of the base  are separate parts that must be screwed together. A base can thus sometimes constitute a complete scope mount assembly, but is most often used to refer to the lower part of a two-part scope mount assembly. The firearm interface which sits on the firearm and to which the scope mount is attached is often called the "base" or "rail". Some types of bases are: Screw pattern on bases. On receivers without an integrated attachment for mounting a scope (for example an integrated Picatinny rail), the base is usually screwed on as a separate part. Such mounts are often model -specific to the firearm, and depend on factors such as the radius of the receiver bridge, the type of screw and the distance between the screw holes. A common fastening method is by screws. These are often metric M3.5x0.6 mm or US #6-48 (⌀ 3.5 mm, 0.53 mm pitch) or #8-40 (⌀ 4.2 mm, 0.64 mm pitch). Many European assemblies use M3.5 screws, such as Sako Optilock, Recknagel and original CZ rings. Since #6-48 and M3.5x0.6 have near identical diameters and almost equal pitch, there is a potential for confusion, and upon mixing the wrong screw will  enter the threads, but will gradually become tighter to screw until the thread is destroyed. In case of damage, the hole must often be drilled and re-threaded, and M4x0.7 or #8-40 may then be relevant alternatives. The Remington 700 Short Action (SA) scope base attachment pattern is particularly widespread, and is for example used on models such as: The Remington 700 Long Action (LA) naturally has a longer distance between the front and rear screw holes, and therefore continuous scope mount assemblies for the 700 LA do not fit on the 700 SA nor the above-mentioned firearms . However, two-piece scope mounts in general interchange for the mentioned models. Bases with a rounded bottom for mounting on a round receiver bridges should ideally have a slightly smaller radius than the receiver in order to provide two points of contact and give a stable attachment. Conversely, a slightly too large radius on the mount will result in just one point of contact and a less stable attachment. In the table below, the radius refers to the curvature of the mounting surface on the receiver bridge. The base is often attached with two screws on the front receiver bridge  and two screws on the rear receuver bridge, but sometimes with several more screws. The hole distances are measured from center-to-center. Some common hole distances are 12.7 mm, 15.37 mm and 21.84 mm (0.500", 0.605" and 0.860", respectively). The two front screws are referred to in the list below as screws 1 and 2, and the front hole spacing is thus referred to as «distance 1-2». In the same way, the rear hole distance is called «distance 3-4». The distance between these is largely determined by the receiver length, and is stated here as «distance 2 -3» Other features. Quick release. Quick release (QR) can refer to several different variants of scope mounts which can be mounted and disassembled quickly without tools. Tilt. In some cases, it may be relevant to add extra inclination to the scope to be able to shoot at longer (or shorter) distance. For example, this is popular for long range shooting, where it is common to use a tilt of 6 mrad (20 MOA). Extra tilt can be achieved several ways, like for example with a tilted Picatiny rail (e.g. 6 mrad tilt), with bases or rings (e.g. 6 mrad tilt ) or with special insert rings (e.g. Burris Pos-Align). Scope height. The height of scope sight can be important for the cheek rest support (often called "cheek weld") in order to gain correct eye placement, as well as for calculating ballistics (e.g. a ballistic table). The latter is particularly relevant at very close ranges (e.g. 15 meters [50 ft]), while at longer distances, such as in long range shooting, the scope height has less impact on the ballistic calculations. The height of a scope sight can be measured in many ways. With regard to ballistic calculations, it is generally only  measured from the center of the bore axis to the center of the scope sight (sightline). With regard to cheek support, several methods are used: On firearms with a picatinny rail, the height is measured from the top of the picatinny rail on the firearm. On most other types of bases it is common to measure from the top radius of the receiver bridge. When the bottom measuring point is determined, the height is then measured up to either the optical center or the bottom of the scope tube (on scopes for ring mounts). The difference between these two measuring  methods is distance from the optical center to the bottom of the scope tube, and usually corresponds to half of the tube diameter (e.g. 15 mm on binoculars with a 30 mm tube).