To do that with a scope I suggest you a different method.
This effect here is for models that reflect everything from all sides - some kind of 3d mirror. A scope only needs the reflection of one side (I think you are talking about its lense).

You have to make a *VIEW which shares the same position as your reflecting part of the scope and which is directed (all the time) at the part that should be reflected.
To show what I mean comment the lines 49 to 65 in the DEM.c file and look at the ball model (this spares a lot of the framerate).
To use this effect on the scope I suggest that you split the model of the scope into 2 parts: the normal scope and its with vec_for_vertex attached reflecting lense. I never did such a thing before though, but it may sound more complicated than it is.
Nevertheless, there may be better solutions.