I was visiting a local gunsmith, a elderly man who has had a long career in gunsmithing and who's opinon I have always respected. As we spoke, he was mounting a very nice scope on a very fancy target rifle for a customer. He cut two small pieces of black electrical tape with a razor knife just slightly small than the rings and lined the rings with that tape. When I commented that was new to me, he smiled and began to explain. However, we were interrupted by another customer and never returned to the subject. So, what's the deal? Is this common and if so, why?