Mounting rails on rifle scopes are much more dependable than rings, to be clear. I do not know why, but they gain little to none popularity overseas. Here, in Europe, rail standards are rising and have already been popular for a long period of time.
The oldest mounting rail standard, known as "LM rail", "70°C rail" or "Prism rail", has been in use for decades. Nowadays, it’s slowly dying out (or it is already outdated, to be fair) and being replaced by modern rail standards, such as Swarovski’s SR rail, Zeiss’s ZM/VM rail or similar. However, LM rail standard was used for a long time and it was used only by European manufacturers (and it was widely spread among them). Its lack of popularity today might be a result of needed drilling during installation of this mounts.
There are still a lot of rifle scopes out there with LM rails which are perfectly working, so it is logical that manufacturers still produce mounts for those scopes. In contrast, there is basically no more new rifle scopes in production with such rails.
At first, LM rail had a prismatic shape in its cross section and the angles of the rail were placed at 70° angle. The rail was 15 mm wide at its base. The problem with these rails was that almost all of these rails were individually fitted and custom finished, because this was the only option to join rifle with the rifle scope. Later, LM rails became standardized, which made mounting process easier (because all such rails got same size and dimensions) and it was no longer important which scope manufacturer produced the scope with its rail.
LM rails are definitely a reliable solution. Therefore, they are durable and robust. Without rings, there is also no pressure or tension on the scope tubes (and no signs on the scope when removing it). A lot of supporting material always result as a good solution with more powerful and hard kicking calibers. Screws that connect the rifle with the attached scope, also work as a recoil stoppers. Last, but not least – achieving the perfect horizontal alignment is simple with the railed scopes (no matter which type of rail is used).
Huge drawback of LM rails is that they required a gunsmith to first drill 2 holes into the mount from side to side through the rail and tighten the rail through them with screws. Also, gunsmith had to carefully placed these holes with exact locations of the holes on the rifle to make a joint with screw. If this part was not made carefully, screws could not have been inserted correctly. If the scope was later put on a different rifle, there was almost always a need for drilling new holes, because old ones from the previous rifle were useless (distances between slots were not standardized as it is with new rails, Picatinny for example).
From economic aspect, this results in higher costs and non-interchangeable parts. We should not forget to mention that beside the fact that there are less and less mounts and scopes with this rail on the market, there is also too much potential for errors with this type of mounting.
In the past, all major European manufacturers were producing and offering rifle scopes with LM rails:
It is interesting that Europe is the only part of the world where mounting rails are popular and on the rise and were already interesting to the buyers in the past decades. Mounting rails still aren’t reaching no success overseas (USA, Japan,...) which seems strange to use – we must admit that there are a lot of advantages when using railed scopes on the rifles.
LM rail is an old rail standard, found mostly on old scopes. However, it was very important in the past and it has showed some guidelines to important manufacturers of rifle scopes on which they developed their own rail standards (that can be found on the market nowadays). For all of you LM-railed-scope-owners there is a good news: manufacturers of scope mounts are still producing mounts for your scopes.