You would need to fork the camera scripts, the things you are looking for are in StarterPlayerScripts.PlayerModule.CameraModule.CameraInput. There is a variable called “local ROTATION_SPEED_TOUCH”, its a vector2 so you can change X and Y sensitivity.
As for changing this variable when you are scoped in, you can do that a number of different ways such as checking for a character attribute like (“isScoped”) or a bindable event.
If you don’t know how to fork player scripts, all you need to do is press “Play” and while in play testing copy the 2 instances under StarterPlayerScripts “PlayerModule” “PlayerScriptsLoader” into clipboard, there is a 3rd for sounds but you don’t need that. Stop the play test and paste what you copied into StarterPlayerScripts, from there you can edit roblox’s player scripts.