If you equip the MX assault rifle which has the under barrel grenade launcher attachment and equip an MRCO weapon optic to it. When looking through the optic you can still see the front iron sight on the first two zoom levels, but its a white color instead of the gray/black natural color.

Note: This only happens when aiming down the sight. Its fine when in first person view without aiming down sight {F20635} {F20636} {F20637} {F20638}