35mm scope rings and mono-block mounts are usually very wide, with at least 20mm width. This way there is a huge contact surface between the scopes and the mounts.
The size of 35mm main tube is a rarity, a kind of compromise done by only a couple of companies: the Romanian IOR and the USA-based Vortex and Leupold. Why did they choose to use this diameter is a mystery by itself. Especially in a time when all other rifle scope manufacturers are producing scopes of the same Category with 34mm tubes. Due to this reason, there are only a couple of manufacturers of mounts that produce mounts with this diameter. Almost all mounts for such scopes are of tactical nature, either in the form of tactical scope rings or tactical mono-block mounts.
The most well-known brands of mounts of this diameter are:
Contessa 35mm Scope mount
The most important thing when mounting a rifle scope with rings is to be aware of the torque needed to fasten the screws in the scope rings. Go over the limit and the structural integrity of the rifle scope will be compromised. Scope rings can also leave a visible mark if not tightened accordingly. The amount of torque should not exceed 2.2 Nm
Lapping is a process that is essentially lining two mounting rings perfectly to one-a-another. It is done with special cylinders that grind off the excess metal.
When the ring is constructed from two near half circles - so if the circle of the ring is split in half, roughly in the ratio 50:50 and such riflescope rings are mounted on two separate mounting bases, then lapping is recommended. If lapping is not performed, the very small offsetting can be present when all the different pieces of the mount come together and when both scope rings are mounted on two separated bases. If the mounting of two high-quality scope rings is done on a single-piece base, then lapping is not needed. This is one of the reasons that Picatinny rails are becoming so popular in recent years.
In the worst-case scenario, the axis that runs through the center of the rear scope ring does not completely match the axis that runs through the center of the front ring. For this reason, the scope, if mounted into such rings is under unwanted structural stress. The most obvious problems that occur due to this reason are less than perfect optical performance, tracking problems, the side focus mechanism can stop working and the point of impact groups can be dispersed.
With 35mm scope mount, there is also quite problematic to get suitable lapping tools, since most of them are produced in 30 and 34mm diameters. It is a good idea to use a ribbon cut from latex/rubber gloves on the lower contact surface in the ring.
Lapping is also not required with monoblock mounts and all mounts are made from one piece of material and alignment of both scope rings is usually perfect. Lapping is also not required with high-quality European mounts since their tolerances are minimal.