After looking at FOB, I do not see how it could be used with TAC X-bows.
Unmodified TACs require the arrow fletching to be 4-fletched with vanes at 60 degree x 120 degree to fit in string slot (area between rail and scope mount).
Modified TACs, those with cutoff scope mounts or scope mount with slice partway down middle, still have bottom rail that FOB would hit.