The iron sight on the sniper rifles (M320 LRR .408 and GM6 Lynx) shouldn't be adjustable like the main optic.
In the case how the optics are designed in ArmA 3 (iron sight on top off the main sight), the iron sight can't be adjusted. If you adjust the main optic on the weapon, you don't move the complete optic. It just moves the reticule pattern inside the optic.
Furthermore it should save the distance you choose. If you switch between the sights, the distance you adjusted is getting set back to 500m (default).