If the front sight is mounted to the barrel, the barrel isn't going to flex unless a rail sytem is also attached to the barrel, and only if you are putting some pressure on it. There are two types of pressures, one is hardware, but is not going to affect the zero, as long as you do not remove the equipment. The second type is hand pressure, espec. pulling down and rearwad via a fowrad pistol grip. If you use a free float such as a SIR or RAS2, they are more rigid than a barrel cause they are designed to be that way, apposed to a gun barrel that is only designed to shoot bullets, and not be a support pipe/closet clothes rail for hanging things. Any external forces (hands, bipods, on & off equipment)touching or pulling in an un-uniform inconsitant manor on a barrel will scew up accuracy, hence Free Float is best all round.
Good shootin, Jack