Free float rail! It provides a solid mount for the optic and increases accuracy by free floating the barrel.
FF rails are not really designed to mount optics on. They are accessory rails.
The only rail that one can be certain will hold zero has to be somehow connected to the barrel or receiver. It has to maintain that relationship with the alignment of the barrel.
By definition, a free-float system is designed to eliminate the relationship between the forearm and the barrel.
If you are looking for maintaining zero and accurate shooting, mounting on the forearm is not recommended.
However, there ARE options.
For a good bit of money, you can get a Predator monolithic rail upper receiver. This has a top receiver rail that extends the entire length of the forearm.
For less money, you can install one of the ARMS receiver mounted rails. I can't remember the number. An added plus is that it has the rear BUIS already.
For significantly less money and with less rail length, you can get a YHM extended riser. It attaches to the reciever and extends 5 inches. However, this YHM rail isn't going make you a scout rifle. It will "cantilever" the optic to give more eye relief.
If more eye relief is all you are looking for, the best solution is a cantilevered optic mount. Larue makes the SPR-E but it isn't cheap. American Defense makes another one that isn't cheap.
Command Arms has a cantilevered optic mount that seems to be getting good reviews for a good price. Aim Surplus sells them.
I hope this helps.
-- John
