The idea would be to use the collimator effect to draw a scope shadow. Like, a black box with a hole in it the size of the scope's FOV, so if your gun is still and the scope is perfectly aligned, the shadow isn't visible at all. When you move about and the scope misaligns slightly, the shadow would (in theory) become visible.
Well, that's the theory at least. I have no idea if it's plausible or not. {F20203}
I've attached an image describing scope shadow, for those who aren't familiar with it.
Now the "3D Scope" do this a little.
unlike the Red dot which have already been poper simulated,the reticle of 3D scope is on the scope it self,so if you look at it slightly from one side, the hit point do will hit the opposite form the aim point.
but the shadow is hard one, realistic optics of lens is impossible for PC now,even with PIP scope,it would be hard to make.