TS, I second the "eagerly waiting" comment above! But I don't understand your comment about inefficiency and lighter stones. The images that I've seen on the web of staff slings don't appear especially unwieldy.
http://www.eriding.net/media/photos/history/romans/051220_pelliott_mp_his_romans... Stones lighter than, say, lacrosse-ball size? The staff sling I was playing with this afternoon was about right for a lacrosse ball. (It was a modified lacrosse stick that I was using.) A rock of that size would not be much heavier than LAX balls are.
But I'm liking the way your shepherd's-staff-plus-one-handed-staff-sling suggestion wraps up the plural question in the Goliath account: "sticks". A minor point, and perhaps unimportant, as Dale's comment notes.
SV, your comment about camouflage--which I referred to in the original post--seems to be based on the conventional assumption that David was using a standard sling. But the point of my post is to ask whether it might not have actually been a staff sling. To your idea, however: if the string/pouch were wound around the staff, it would not necessarily have been recognizable as a sling. So maybe we're not that far apart?
Or again, anyone, what about the combo idea--a staff/staff sling?
Yes, the staff sling idea is indeed a big if. But hey, I figure, why not make exegetical history?! (Thanks to all for indulging me!)
(Just to complicate things: Psalm 23, usually attributed to David, mentions TWO shepherding "sticks": "Thy rod and thy staff, they comfort me.")