Stadia lines in a scope are used to measure the shoulder width of the enemy and it will give you a rough estimate of the target range and where hold off will be for a certain bullet.
For instance, the ACOG is used by the US Military mainly on M4s which have certain ballistics for that specific bullet. Whether the scope is manufactured accurately for the type of round we use is unknown by me, but I understand its purpose. ^
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The concept is pretty simple. If the enemy's shoulders are as wide as the #3 stadia line, the target's estimated range is 300m. As long as the scope resembles the specific round fired from the rifle that it was created for, with the exact velocity and good reloading, the round should impact on the #3 stadia line.
This is a complicated project because the bullet physics and the scopes zoom may have to change. And I'm not sure how accurate their distance is in the game, which can make it harder to achieve realistic range estimation/target shoulder width relation to each other. {F18247}