"They are undoubtedly Bludgers rather than cannon balls. The faint indentations of magically reinforced Beaters' bats are visible and one can see the distinctive hallmarks of manufacture by a wizard (as opposed to a Muggle) — the smoothness of line, the perfect symmetry. A final clue was the fact that each and every one of them whizzed around my study and attempted to knock me to the floor when released from its case."
-- Agatha Chubb (QA6)
Agatha Chubb is a witch and expert in ancient wizarding artifacts. While researching Quidditch practices of the 16th century she actually discovered twelve lead bludgers in the peat bogs and marshes of Great Britain and Ireland (QA6). She used her finds to show how modern wizards changed from lead, which was too soft to make a proper bludger, to iron bludgers (QA6).