Do you guys put your front BUIS on the very end of your rail or on the gas block? What are the advanatages over one or the other? Does it matter if you're going to co-witness with a red dot?
If the gas block is pinned, on the gas block. If the rail is top tier and bolts up solid to the gun, mounting it there is fine. FWIW, the preferred method to mount a light is in front of the BUIS or FSB in the 12:00 position. The BUIS is usually placed directly behind it. A Surefire X300 works well there.
Glocktogo pretty much summed it up. Never on non-free floated 2 piece rails in my book. Has to be a larue, Daniels, knight RAS or like. And only on pinned gas blocks. You just want a solid mounting point bottom line.