I need an optics riser for my flattop 20HBAR and I'm considering an ARMS 38, since this will give me BUIS also.What I'm wondering is this...will my ocular clear the #40 sight in the rear or not?  I've seen posts that state that the 40 is too high to sit under a scope, but I'm thinking they were probably mounting the scope to the flattop itself.  I would go with a 36 and a Knights or Mad BUIS but the 36 and the GGG rails both cover the dovetails behind the riser.  Thanks all.

that will depend on what scope you are using and what rings you are using.

In the folded down position, the highest point on the BUIS rises about 1/2 inch above the top of the #38's pic rail.  You'll have to do the figuring yourself regarding where the scope will set in relation to the BUIS, size of the occular and height of rings used.