I saw these pop up in eBay and did a bit of background on them before I bid, the attractive part being the start price of £20 and the fact that having paid a lot of money for the scope new, I normally buy s/h. I already had a pair of very good quality steel rings that mic'd up at 32mm, as many will know this is not ideal as the S&B tube is 34mm. They fitted fine, did not pinch the tube, and dialled in with little problem. The rub was that ,despite them being 20 moa rings, there was still another 28moa of elevation adjustment left after zeroing, so not really getting the best from the scopes' spec. Now, the likelihood and the opportunity of getting to use or having to dial in that 'lost' 28moa Is going to be rare but the OCD clicked in and the gadget need!
The principle behind these rings is that the mount heights (these are the 'low' version) are both the same base to bottom of ring but they have inserts which clamp inside the ring which allow either 0,5,10,20,25,30,40 moa of adjustment. It does this by utilising elliptical interior insets of 0 moa x2 and +\-5 moa +\-10 moa and +/- 20 moa of which there are two sets. The extra range is achieved by combining say a -20moa ring at the bottom of the front ring and a +20moa at the top (they MUST be used in matching pairs +/-) and say a +10 at the bottom of the rear ring matched of course by the paired -10 at the top, this maintains concentricity and gives a total of 30moa, hope that is clear, if not they are well covered off on the web.
So, to try and get at my elusive 20mm I decided to buy the rings as, even if the adjustment principle was unusable, they are well manufactured by a reputable name and the inserts, which are plastic, would be kind to the tube. In the end nobody else bid on them so I got them for £20, bargain, I hope!
Having read a bit more the first thing I did was to mic the inserts as I had read that some sets had gone out with the shims mis marked, these were fine. Now let me say, THESE ARE FIDDLY, the other thing to bear in mind that if the rings are not set equally to the horizontal axis of the ring they will of course act as a collimator therefore affecting adjustment on the horizontal axis I.e. Windage. I went through the standard scope mounting procedure to get the eye relief right and initially set it up with the 20moa inserts in the front ring, - at the bottom + at the top and the 5moa at the back + at the bottom and -at the top this should have given me roughly the missing 28moa. At this time the ring distance between centres was 4 inches. The distance between the illumination adjuster and the bell to the objective lens on this scope is quite limited with, of course, the reticle housing in between. Using this setting and using a laser bore scope this still did not give me full access to the adjustment range leaving a missing 10 moa. So I disassembled all over again and put the 20moa adjusters in the rear again +20 at the bottom and -20 at the top so the reverse of the front inserts and this should be 40 moa total. After reassembly I was still missing 5moa! Again reference to the wonder of the web and the instructions that came with the scope I again disassembled the rings and moved the rings closer together, now 3.50 inches apart and BINGO! Dialling the scope down to the bottom stop I only had to come up 1moa, 4 clicks on this scope, to match the laser dot and then set the zero stop in the turret. Now you know why I say FIDDLY.
As as yet I have not shot the rifle and we are away on holiday soon so I will post my finding from the range. The fly in this might be the effect of the inserts on the windage setting which only seem to be 4moa off the original setting but if I want to centralise the windage adjustment and see where the poa is then and adjust the eclipse of the inserts then this will affect the elevation as at the moment, relative to the vertical axis, I have maximised the down angle, this obviously changes if the inserts are rotated to adjust for the horizontal axis.
I hope you find this post of interest, or at least an advice that unless you have a few hours to spare and reasonable degree of patience these may not be for you!