The 20x Ballistic Scope is an optic seen in the customization screen on Battlelog. It was revealed when customization was added to Battlelog. This attachment is only available for the DICE developers.
Theoretically it will only provide 1700% magnification, unless DICE developers have tweaked the zoom to an impressive 2000%, but is most likely not the case, because they would then have to redo the entire zoom magnification system for just one single item. Technically speaking, a 12x Ballistic Scope merged with a 8x Rifle Scope for 1700%, or two 12x Ballistic Scopes merged together for 2000% zoom. For instance, the 12x Ballistic Scope provides 1040% zoom.