Zeiss Inner Rail, often simply called Zeiss rail, is a ringless telescopic sight mounting system introduced by Zeiss in 1990 as an alternative to traditional ring mounts. A patent was granted in 1992, and the patent expired in 2008. The mounting system is now also offered on sights sold by other major manufacturers, such as Blaser, Leica, Minox, Meopta, Nikon, Noblex (formerly Docter), Schmidt & Bender and Steiner. It has therefore, in some sense, become the industry standard for scope mounting rails. The system has so far seen most use on the European high end market.
Before the Zeiss rail, many European scope manufacturers used to offer a single type of standardized ringless mounting solution known as standard prism. This mounting solution was also known under names such as exterior rail, 70° prism rail or LM rail (Light Metal). Compared to ring mounts, this type of mounting rail permitted mounting without putting compression on the internal mechanics of the scope. The system also allowed the shooter to place the scope at their preferred height and correct eye relief (distance to the eye), as well as the opportunity to easily move the scope between different firearms. However, the standard prism had an aesthetic drawback in that the scope rail had to drilled on the side for attachment screws. In case the rifle scope was to be used on different guns, new holes often had to be drilled. A motivation for developing the Zeiss rail was thus to avoid such drilling.
The Zeiss rail system was introduced in 1990 as an option on all Zeiss ZM/Z riflescopes, the top of the line riflescope offered by Zeiss at that time. The system was later offered on the new top of line VM/V models. For these reasons, some sources have referred to the Zeiss rail system under names such as Zeiss ZM/VM rail or Zeiss M rail. Names such as Zeiss Integral rail, Zeiss 45° rail or simply Z rail have also been used.
Compared to the older prism rail, the Zeiss rail does not require any drilling, and therefore provides easier mounting to the user, as well as improved aesthetics. Compatible scopes have an internal dovetail rail where two or more 45 degree wedge nuts can be slided in. For example, EAW uses wedge nuts with M5 or M4 threads. The scope mount is then attached using simple hand tools like a torx key or hex key to a torque between 4–5 N⋅m (3.0–3.7 lbf⋅ft).
Drawing of scopes with Zeiss rail (left) and ring mount (right), both with picatinny receiver interface. https://handwiki.org/wiki/index.php?curid=1903002
Wedge nut with metric M5 threads. https://handwiki.org/wiki/index.php?curid=1441079
The Zeiss rail system can be found on some models from scope manufacturers such as Docter, Leica, Minox, Meopta and Schmidt & Bender, and sometimes only on high end models. Often the manufacturer will offer these models in two variations; one for traditional ring mounts, and another for the Zeiss rail mount.
There are also examples of rifle scopes that thave been sold exclusively for Zeiss rail mounts (i.e. no option for a ring mount version). These include the Zeiss Victory Diarange laser rangefinder scope and the Zeiss Varipoint iC models. In late 2017, Blaser released their Infinity iC (illumination Control) line of scopes which also only uses the Zeiss rail system.
Aftermarket mounts compatible with the Zeiss rail system are offered by several well known manufacturers such as Blaser, EAW, Henneberger, Innomount, Kozap, MAK, Recknagel, Rusan, Uronen Precision, Virtus, and Ziegler. Both two-piece or monobloc mounts are offered.
Compatible mounts are offered in different configurations, depending on the mounting opportunities on the firearm receiver. Examples include Picatinny, Weaver or different types of claw or swing (pivot) mounts.
Competing standards to the Zeiss rail include the Swarovski SR rail and Schmidt & Bender Convex rail. The three systems are not compatible. While the Zeiss rail has a stepless dovetail shaped mounting surface, the Swarovski SR rail has a finely toothed rail, and the S&B Convex has a smooth convex rail.
Swarovski applied for patent on their SR rail system in 2002, and introduced to the product to the market in late 2005. The Swarovski SR rail is also used by Kahles, a Swarovski subsidiary.
The Convex rail has been offered by Schmidt & Bender since at least 2005, and has also been marketed under the name LMC (Light Metal with Convex rail). Contrary to the Zeiss and Swarovski rails, which ensure a levelled reticle, the S&B Convex rail instead allows the user to tilt the reticle up to 1° (60 moa; 17.5 mrad) to the left or right. Since 2016, Schmidt & Bender has also offered the Zeiss rail system as an option on some of their hunting scope sights under the name LMZ (Light Metal with Z-rail).