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Webster 1913 Edition

(gŭn′stōnˊ)

Noun.

A cannon ball; – so called because originally made of stone.

[Obs.]

Shak.

Noun.

A stone used for the shot of cannon. Before the invention of iron balls, stones were used for shot.

gunstone ‎(plural gunstones)

(obsolete) A cannonball.
(Can we find and add a quotation of Shakespeare to this entry?)