Access Stencil Buffer on opaque material [Alternative]

I’m trying to cull mesh on weapon inside the view of scope. Tried using scene capture 2d just like what they did on call of duty (Dual Render) but it gives me really bad performance. Trying to switch on depth/stencil mask, the problem is, opaque material wont access scene textures that contain depth masks.

You do not have much alternatives here, but due to the fact that the scope itself is a simple shape, you can make your weapon a masked material, and calculate if each pixel is behind the scope glass analytically. Check if the pixel is within a cone, defined by its apex at camera location and cross-section at scope glass. If so, set opacity to zero.

Im stuck on getting the cross section part at scope glass. Can you visualize to me what do u see on that?

Did you get any solution for this?