A couple of weeks ago, my FWB 601 arrived. Carel included an Eagle Eye diopter .05 lens with it. I had never used one before, but I really like it. It is a big help at 10 meters. My tired 68 year old eyes do much better with the lens. I think that they are still available somewhere.
If my 300S had the same lens attachment, I feel that I could shoot it with equal accuracy. I do enjoy both rifles.