Telescopic sights with a mounting rail under their main tube are almost always made by European producers and meant for hunting. Rail mounting brings these advantages:
Fewer tensions on the scope
More reliable mounting
Mounting without marks on the scope
Mounting of such scopes with modern rail systems is easy and can be done by less experienced gunsmiths or even by users. Selection of mounts is limited to premium European manufacturers, and such mounts are usually more expensive than ring alternatives.
Most common mount rail systems are:
LM rail: a traditional prism rail under the scope. This standard was used in the past by all major European manufacturers and is commonly found on old hunting scopes. It is still used in limited production by S&B, Docter, and Kaps on their Classic lines of scopes. Mounting of scopes with LM rail is the most demanding of all rail systems since it demands drilling through the rail. Mounts made for LM rail have screws that go through the rail’s cross section.
Zeiss ZM/VM rail: a modern standard used by the biggest number of producers. The companies that use it also call this rail standard: 45° rail, Zeiss M rail, Docter Z rail, Meopta MR rail, Minox is rail, Leica rail, or S&B LMZ. There is no drilling needed when mounting such scopes and usually 2 or 3 internal elements are inserted into the rail.
Swarovski SR rail: the main feature of this rail system is small ribs (recoil stoppers) that prevent movements of the scope during recoil. The system is used on Swarovski and Kahles scopes, and 2 or 3 inserts in the rail are needed when mounting. All high-quality mounts made for SR rail have only one point, where small ribs are fixed, usually on the front mounting element.
S&B Convex rail: this rail system is used exclusively by Schmidt & Bender on their hunting lines of riflescopes. The basic principle of internal parts inside the rail and outer parts is the same as with Zeiss ZM/VM rail standard; however, the shape of rail is convective and not made of straight lines.