4Game

In short, all scopes put on weapons will be accurate … with the default ammo type for the weapon. The default ammo type isn't shown in the item description in-game, however you can find the information on the wiki (Example with the Mosin), on the right side you'll see small table showing that its default ammo is 7.62x54mm R LPS gzh).

Now, if you use a different type of ammo, because their weight, velocity, and other characteristics are different, it will not shoot correctly for the scope. For example, if you use a heavier and slower ammo, the drop will be significantly increased making you shoot way lower than you aim at, even with correct zeroing. If the ammo is lighter and faster, then you'll shootway higher instead.

Therefore, either stick to the default ammo, or you'll have to learn and manually adjust for the difference. Hopefully this solves the mystery for other people beginning with snipers like I do.