Adjustable trigger
The ability to adjust the features of the trigger triggering of the rifle — first of all, the idle (before triggering) stroke of the trigger and the force on the hook. This allows you to optimally adjust the characteristics of the weapon to the personal preferences of everyone: for example, it is usually difficult for beginners to shoot with a tight trigger, but for trained shooters, a too soft hook that “falls through” under the finger may be inconvenient.
Scope mount diameter
The diameter of the mounts that the complete optical sight of the rifle is equipped with. For optics in general, see Sights. And the diameter of the mounts is the diameter of the rings in which the tube of the sight is fixed. These rings are mounted on brackets, with which the entire structure is attached to the rifle.
In the vast majority of cases, the diameter of the rings is a purely reference parameter that does not affect the choice. Before buying a rifle, this data may be needed if the "household" already has another optical sight, which may be used instead of the standard one. And even in such cases, usually, it is easier to use the "native" mounts of a spare sight than to install it in the rings from the complete optics.
Also note that the vast majority of pneumatics with optical sights use mounts of a standard diameter — 25.4 mm. Other values (for example, 19 mm) are extremely rare.
