It looks like the base of those rings is set and not adjustable. So, if you back the screw all the way out and they still won't slide on, you'll have to open them up some. I would see where they are binding up or not allowing them to slide on, and try to file down a little on the base of the ring to allow it to fit. I've done this before on some one piece bases like this before. If it's not fitting, it's not fitting by much.