  Medieval bullet mold. 
According to the eBay seller:Circa 15th-17th century AD. Medieval bronze bullet die. The die was used to cast bullets for muskets. Beautiful green patina. Length: 23 mm. Well it's obviously only half of the mold, you can see how it has index pins meant to fit with the other half to form a complete sphere mold.  I'm a little skeptical about the dates, it looks a tad too sharply cast to me, but what do I know about ancient brass. 
Source: eBay seller ancientcaesar 
Contributor: Theodore Gray 
Acquired: 2 April, 2009 
Text Updated: 7 April, 2009 
Price: $10 
Size: 1" 
Purity: <80%
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