You guys act like there are no other .308 rifles with a good chunk of mass cycling back and forth. Ever see the HK G3 stripped down?
Now, granted that there are scopes made specifically for the AR rifle in 5.56. Those may not fair too well. But there are plenty of reasonably priced scopes made to handle calibers much more powerful than the mere .308 cartridge.
Also FN shooting teams don't use cantilever scope mounts. They use the good old fashion rings with the picatinny bases.