Burris is the only one I know of with the plastic inserts. Jewel has a set with aluminum inserts. Not trying to hi-jack the thread but, a friend and I've been working on a set of rings and base combo for several years. The pic here is still a prototype, the finish on them will be a clean bead blast or possibly anodized to your choice of color.
I'm getting closer to start manufacturing these, if I can get enough interest again. Rear base tapers up and front base tapers down. Aluminum inserts for either 30mm or 1" tubes. Allows you to loosen the ring caps, loosen the clamp on the ring and slide either the rear up or the front down or a little of both to get your 100 yard zero as close to bottom as possible, then tighten with no lapping and perfect scope alignment. I have a 100 yard zero on my .338-.378 (3100 fps) with a 8.5-25 lupy and have enough internal clicks to go out to around 2100 yards.