Aut cum scuto aut in scuto
Peltasts were a key element of Greek light artillery. They'd go out with slings and ammo like that one and take on the slowly moving, heavy (phalanx) infantry.
How big is it?
Size: unsure on the exact one pictured.From a photo archive of similar projectiles, I'd say 1.25"-1.5" looks standard.http://www.portal.cig-icg.gr/node/579
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Scientific American, back when it published science articles, once had a story about slings and arrows. Slings, in particular, were quite effective against the armor of the day, and the stones could penetrate a considerable thickness of it. Goliath was hopelessly outgunned and doomed to defeat.