Just a comment on my "rails on top of rails" solution as "life is full of trade-offs.
I'm sure most would look at "rails on top of rails" and say "ooo - why would anyone ever do that!"
My original reason is because unlike true "olde school" types, I move my scopes around. Not as much as I used to, but I still move them around. And if I use cantilever uni-mounts for the stoners, then if I move that scope to a bolt gun, I have to reverse the scope in the cantilever mount so it is reverse cantilevered. Then if I move it back to the stoner, I have to reverse the scope in the mount again, back to cantilever.
Well, while I like to move my scopes around, I do NOT like to take the scopes out of the rings and put them back in the rings.
So to avoid having to re-ring the scopes, I switched to using real rings for all the scopes, and setting them up to run on bolt guns and then using the rails on top of rails so the scopes are not "bridging" the receiver and forearm rails. Such a bridge, does not always work as the rail "gooves" on the forearm may not be spaced to mate with the bridging item. And the cant of the forearm rail may not be identical to the receiver which means something somewhere is twisting if you torque the rings down on to both the receiver and the forearm (of the stoner). And that something might be the scope. And that does not sound like a good thing. So using the receiver rail on rail, to avoid bridging to avoid twisting the scope.
The forearm rail on rail is then to allow the clipons to be mounted at the same height as the scope.
So essentially I'm setting up a "cantilever" that is not part of the scope mount itself.
The trade-offs are more weight and the rails on rails are not as solid a platform as the receiver itself, so I have to check the torques of the rails on rails to make sure they are not coming loose.
Another option, is to get a longer forearm rail. That is replace the forearm rail with one that "hangs out over the front of the forearm (or get a longer forearm if possible) If it doesn't need to be too long and you make it out of steel, then it should be solid enough and not be floppy. That might work for the case in the pic.
But yes, getting everything mounted "optimally" for your gear and your shooting habits is a non-trivial exercise. Especially if you use clipons and especially if you move your scopes around. It is a never ending challenge for me, since I also get new gear and sell old gear from time to time but I don't mind, it is part of the game
I use Farrell Mfg of Warsaw, MO for my customer rails and scope bases. They're making me an extra long scope base for my Barrett M-99 right now, so it can handle clipons.
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Here is the 5.56(18) with its relatively new "rails on rails" setup (unpainted yet). Note this is the setup I had outside, the 14 on the rear is for zeroing the LRF without the clipon on. Then the 14 was removed and the clipon mounted. But I took everything outside and inside all attached to the gun, so the spare parts wouldn't be flopping around.
