If you look for the addons from which those ironsights are coming from :
The M249 and the M16 ones are made by Earl and are included in the Marine Assault Pack
The aimpoint one i think is made by Benus and comes from the FFUR replacement mod, but i don't know which version exactly.
If i remember well, the last one for the M60 comes from InQUisiToR high detail weapon pack
Most (if not all) the replacement mods are using customised ironsights, either those or different ones, and several standalone addons are using them too, like Laser weapons for his units.