ali-a opened a weapons page on the site of call of duty but i could'n find it help!
you must expert all weapons in a class to get the diamond camo
you have to prestige your weapon twice i think?? and get all camos on the weapon then you'll get the diamond
you must expert all weapons in a class, "ALL" the ARs not just 1 of them, to get the diamond camo. Hint, i found it was easier to do the no perks or attachments at the same time and use shock chargers when doing the semi-auto weapons.
How old was the video you saw this in? You could do that with Elite when it was still around.