• Quoted Excerp from http://en.wikipedia.org/wiki/Telescopic_sight#Mounting

"Mounting

Colt Python Silhouette, with 8-inch barrel, factory scope, and case — 500 made in 1981 by the Colt Custom Gun Shop.
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. It is always wise to buy telescopic sights that provide a decent adjustment range, preferably at least 60 MOA or more.[19]

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 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[20][21] 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.

Scope rings
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 3/4 inch (19.05 mm), 22 mm, 1 inch (25.4 mm), 26 mm, 30 mm and 34 mm. 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.

Scope mounting rails

Schmidt & Bender Classic 4x36 with mounting rails fitted with a Suhler claw mount on a Gebrüder Merkel Drilling. Manufactured in May, 1985.
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:

Standard prism
Zeiss ZM/VM, also used by DOCTER
Swarovski Optik SR
Schmidt & Bender Convex
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.

Rail interface systems

Telescopic sight fitted with scope rings on a Picatinny/MIL-STD-1913 rail mounted above the receiver of a sniper rifle.
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.[22] 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.[23]

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."

Other Links:

http://en.wikipedia.org/wiki/Weaver_rail_mount
http://en.wikipedia.org/wiki/Picatinny_rail