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Try the Samco M33 handguards. They replace your existing handguards and have the rails on top and bottom of the handguards. I have a set with an Eotech 511 mounted no my AR and the co-witness lines up exactly.
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The M33 handguards are fine for mounting a light or vertical grip, but not optics. The M33 handguards are probably the worst way to mount an optic in front of your carry handle. Your optic will not hold its zero as the handguards will shift around if you put any kind of pressure on them.
The reason both sides of the foubus handguards have rails is they have a one cavity mold, the mold spits out one half of a hand guard for every cycle...it isn't there to mount optics. If Fobus reccomends doing so, I'd say its pretty irrespsonsible on their part.