http://www.guampedia.com/category/22-ancient-guam/entry/380-slingstonesThis is a great article. Some choice highlights:
One account by Father Peter Coomans in The History of the Mariana Island Mission for the 1667-1673 period describes the weapons as such:
Their offensive weapons include the sling, which they aim very skillfully at the head. Out of small ropes they weave a sort of net-bag, in which to carry stones with an oblong shape, some formed out of a marble stone, and others of clay, hardened in either the sun or fire. They whirl and shoot those so violently. Should it make an impact upon a more delicate part, like the heart, or the head, the man is flattened on the spot. Then, if envy would make them want to burn a house from a adistance, they would stuff the perforated side of it with tow burning with a very ferocious fire, which, with a swift movement became a flame, and sail away to seek shelter in enemy houses.
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The vast majority of Marianas slingstones are hand sized, weighing from 40 to 80 grams, though a few exceed one kilogram in weight. These are thought to have been ceremonial but perhaps some served utilitarian purposes, such as as canoe breakers as seen in Tahiti and Hawaii.
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Guam, however, is the world’s only political entity that features the slingstone on its seal and flag.