Andean Sling is a technology in Age of Empires II: The Forgotten that is unique to the Incas and can be researched at the Castle. Once researched, it removes the minimum range from Skirmishers, Slingers, and Genitours.
Andean Sling is a useful technology, solving one of the main weaknesses of Skirmishers and Slingers. Any respectable Incan army will be using Slingers, and often a few Skirmishers, so generally it is a technology worth researching. If given proper support by melee units, it becomes irrelevant, however, as the troops will never have to fight at extremely close range. It is still generally worth researching, so that in case the melee units die or something slips past them, the range support can easily hold its own.