Ric, as Andre said mine slip over the outside of the scope . I measured each of my scopes and then go to my PVC bin and find what I need. Sometimes you will have to take them to the lathe, if you have one. As far as keeping them on the scope I've found that lining the ID where the cover meet's the scope with black felt work's great for me. It make's a snug fit and will not scratch the scope, a couple dab's of super glue around the ID where the felt go's hold's the felt in place. Before the black felt go's on I shoot it with flat black paint and seal it with flat clear, clean with spirit's first to get any finger oil's off. I made my own after looking at the prices of them . ;( Ed