I think I understand what your saying about the focal point. As it relates to the G-36 or any other rifle for that matter, if the iron sight or reflex sight were mounted way up high in relation to the barrel, then the sight would only be accurate at a specific distance and would shoot too low at any further range, or shoot way too high at a closer range because the sight is way off center of the projectile's line of travel, in a vertical relation. However I think that this is negligable because the iron sights would only be used at close range, and therfore would only have to be calibrated for a range of 100m or less. Anything further out would simply require you to switch to the scope.
Anyway, I'm still wondering if it is possible to have multiple gunsights simply by switching between semi-auto and full-auto. Does anyone know if it's possible? As for whether or not the BIS G-36 was modelled to have a scope and iron sight; I'm not sure. Right now I'm using INQ's retextured G-36 in place of the BIS model, and INQ's model seems to have two scopes, not a scope and iron sight. However someone on another forum informed that the G-36 does infact have a scope and ironsight. Whether or not this info is accurate, I don't know....