Just gave it a quick google (as I wasnt familiar with the project) and it looks like they use PIP. You can zoom the scope in variably and the rest of the screen stays at the same FOV.
Yes a scope will always be in position ‘X’ on a given weapon, but for example in the case of an ACOG. The ACOG will be positioned differently on every gun potentially, now if you want to just make a per weapon mask with only ACOG in mind, sure it will work, but im talking about every gun having the potential to add multiple scopes.
Games like COD/BF clearly handle this with a dynamic solution, im pretty sure what Chosker said is the way, though I dont know that for sure.
I am 100% sure I am not using a render capture, its too expensive. And I dont think vertex paint/making a manual mask will work. It just wont work for every scope.
If all else fails ill try to make a manual mask obviously, I just see it as a solution that will definitely work but isnt great really…