Iron sight/Scope switch

[Opium]

I am unable to switch between scope and iron sights, what is going on here?
Yes i know its C and i've reset my controls to default several times with no effect.

[AfterDune]

Not all weapons have this - what weapons are you trying?

[mangeface]

Weapons that have BUIS will have a letter next to the fire selector in the bottom right corner of the screen, either a B or S. You cannot switch between the scope and BUIS while sighted in, you have to 'unsight' you weapon, switch to either the BUIS or scope and sight back in.