Every few weeks new thread pops up wishing for rifle scope or moar gunz or this and that.
I think an addition of aperature sight would be best of all worlds.
Historical; won't make game "stupidly easy"; rare item to discover; additional layer of skill for the rifle; very much in the spirit of TLD.
How it works for the player: flipped down to use standard notch+post sight. flip up, aperature sight with windage and elevation adjustments using the arrow keys. no deteriorating condition on its own, maybe slightly increased rifle deterioration to account for the more complex device. One spawn per game, but not always. On a side note Stalker should spawn a rifle too once in a while, not always. Just one, and a lot less ammo, a box or two the whole game.
How it works behind the scenes: elevation adjusts raycast length from say 100m to 300m. while aiming the raycast can be infinite to determine what the target is. When weapon is fired the game sets raycast length to player setting and compares it with actual distance between player and target. if the two values are within say 5% margin (with some variation depending on difficulty), the player scores a hit, with increased chance of critical. Or the chance of critical can be linked to how close the estimate is to the actual distance. e.g. 0% error = 100% crit; 5% error = 50% crit; 10% error = 0% crit. If the difference between the two values is greater than the margin it's a miss. Distances less than 100m use regular hit detection mechanics whether using aperature or notch sight. Reduced sight drift when using aperature sight, or multiply the sight drift to the inverse of raycast sight distance. Drifting is still a factor, but not as much as with basic notch+post sight.
Stretch goals: Windage adjustment for the windy days. Prone and/or braced firing position.