Im not doing it using fancy camera captures, im literally lining the first person camera up with the scope and trying to not see the barrel through it when reducing the FOV. Its not possible even with a huge zoom.
Regarding the mask based on the scope position, that is what im trying to do, but there are many different scope weapon combinations and im trying to make it so that its masking in realtime as the gun moves around rather than just like lining it up perfectly and having a perfectly matching circular mask to do the work. Its just too crude to do it that way.