If you can't find the 40 MOA rail, take a look at the Barrett ExRings. Front ring has a pivot and rear ring allows you to select either 15 MOA or 40 MOA with precision. On a 20 MOA rail that would allow 35 MOA or 60 MOA. For a rail I'd check out Ken Farrel. As rails go, they really don't get much more durable or precise. I had one of his rails on a Rem .308 with the adjustable recoil lug. Solid as they get.
https://barrett.net/accessories/optics/exrings/
As with anything "Barrett", there is a price to pay. With the added flexibility of dual angles that gives one a lot of flexibility that you'd have to have two rails if using other products.
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