Rails definitely flex this is why when using irons, and you sling up tight on a HG mounted front sight, you will notice a POI shift. As long as it's not flexing enough for the gas block to touch the rail your probably fine. So in a nutshell it's almost all barrel flex we are seeing and it's normal to a degree. If the barrel was completely flex free and it was all rail flex then there would be no point in free float rails since external pressures wouldn't be able to affect the barrel. The shorter and heavier the profile(thicker barrel) the less barrel flex. The longer and thinner the profile the more barrel flex.

The amount of flex is contingent upon two factors, the barrel length and profile. The whole point of Free Float rails is to keep external pressures off of the barrel since it's so sensitive to flex. They said that what we are all seeing when we press on the barrel from side to side is mostly barrel flex and not much rail flex.

Either way, BCM and KAC both replied with pretty much the same answer. My rifles were shooting just as accurate from day one to thousands of rounds even with this flex, so I doubt that the barrel nut was loose. I ended up emailing both KAC and BCM about this since people were saying that my barrel nut must be loose or my rail has too much flex. My BCM and KAC are lightweight profile and my LR308 has a heavy profile.

308 AR barrel didn't budge at all in comparison. Well this intrigued me, I tried to flex my barrel on my BCM and my KAC rifles and both of them had flex.

There was a thread a few months ago about long rails and the possibility of flex affecting accuracy.

Free float rail ar15 dan