The hair deal is pretty common knowledge. Just reading it, its spelled with an I not an E. That's how I knew.
I interpret the Bible pretty literally. However, even with the best possible slingers, there is still going to be some variation in accuracy because of ammunition variation.
While weapons are used often in the scripture, the word 'accurate' like we would use it today isn't actually used in relation to weapons, only with scales and information.
http://www.biblegateway.com/quicksearch/?quicksearch=accurate&qs_version=NIVI'm not an expert in Hebrew, but my interpretation is that this is the old way of saying 'really accurate'. Similar to how we would say 'he can shoot the wings of a fly'. Kind of like an idiom. Be really careful interpreting scripture that way with other things, and consult your Dad on it before you take it as absolute.
I'm sure they were still really great slingers though, just like David's Mighty Men were great warriors.